
pywws.Plot
**********


Introduction
============

Like Template.py this is one of the more difficult to use modules in
the weather station software collection. It plots a graph (or set of
graphs) of weather data. Almost everything about the graph is
controlled by an XML file. I refer to these files as templates, but
they aren't templates in the same sense as Template.py uses to create
text files.

Before writing your own graph template files, it might be useful to
look at some of the examples in the example_graph_templates directory.
If (like I was) you are unfamiliar with XML, I suggest reading the W3
Schools XML tutorial.


XML graph file syntax
---------------------

Here is the simplest useful graph template. It plots the external
temperature for the last 24 hours.

   <?xml version="1.0" encoding="ISO-8859-1"?>
   <graph>
     <plot>
       <subplot>
         <title>Temperature (°C)</title>
         <ycalc>data['temp_out']</ycalc>
       </subplot>
     </plot>
   </graph>

In this example, the root element graph has one plot element, which
has one subplot element. The subplot element contains a title element
and a ycalc element. To plot more data on the same set of axes (for
example dew point and temperature), we can add more subplot elements.
To plot more than one set of axes (for example wind speed is measured
in different units from temperature) in the same file we can add more
plot elements.

The complete element hierarchy is shown below.

   graph
       plot
           subplot
               xcalc
               ycalc
               axes
               style
               colour
               title
           bmargin
           yrange
           y2range
           ytics
           y2tics
           ylabel
           ylabelangle
           y2label
           y2labelangle
           grid
           source
           boxwidth
           title
           command
       start
       stop
       duration
       layout
       size
       fileformat
       terminal
       lmargin
       rmargin
       xformat
       xlabel
       dateformat
       xtics
       title


graph
-----

This is the root element of the graph XML file. It does not have to be
called "graph", but there must be exactly one root element.


plot
----

Every graph element should contain at least one plot element. A
separate graph is drawn for each plot element, but all share the same
X axis.


start
-----

This element sets the date & time of the start of the X axis. It is
used in the constructor of a Python datetime object. For example, to
start the graph at noon (local time) on Christmas day 2008:
"<start>year=2008, month=12, day=25, hour=12</start>". The default
value is (stop - duration).


stop
----

This element sets the date & time of the end of the X axis. It is used
in the constructor of a Python datetime object. For example, to end
the graph at 10 am (local time) on new year's day: "<stop>year=2009,
month=1, day=1, hour=10</stop>". The default value is (start +
duration), unless start is not defined in which case the timestamp of
the latest weather station hourly reading is used.


duration
--------

This element sets the extent of the X axis of the graph, unless both
start and stop are defined. It is used in the constructor of a Python
timedelta object. For example, to plot one week:
"<duration>weeks=1</duration>". The default value is hours=24.


layout
------

Controls the layout of the plots. Default is a single column. The
layout element specifies rows and columns. For example: "<layout>4,
2</layout>" will use a grid of 4 rows and 2 columns.


size
----

Sets the overall dimensions of the image file containing the graph.
Default (in a single column layout) is a width of 600 pixels and
height of 200 pixels for each plot, so a graph with four plot elements
would be 600 x 800 pixels. Any size element must include both width
and height. For example: "<size>800, 600</size>" will produce an image
800 pixels wide and 600 pixels high.


fileformat
----------

Sets the image format of the file containing the graph. Default is
png. Any string recognised by your installation of gnuplot should do.
For example: "<fileformat>gif</fileformat>" will produce a GIF image.


terminal
--------

Allows complete control of gnuplot's 'terminal' settings. You may want
to use this if you are plotting to an unusual image format. Any string
recognised by your installation of gnuplot's 'set terminal' command
should do. For example: "<terminal>svg enhanced font "arial,9" size
600,800 dynamic rounded</terminal>". This setting overwrites both size
and fileformat.


lmargin
-------

Sets the left margin of the plots, i.e. the distance from the left
hand axis to the left hand edge of the image area. According to the
gnuplot documentation the units of lmargin are character widths. The
default value is 5, which should look OK in most circumstances.


rmargin
-------

Sets the right margin of the plots, i.e. the distance from the right
hand axis to the right hand edge of the image area. According to the
gnuplot documentation the units of rmargin are character widths. The
default value is -1, which sets automatic adjustment.


xformat
-------

Sets the format of the time / date xtic labels. The value is a
strftime style format string. Default depends on the graph duration:
24 hours or less is "%H%M", 24 hours to 7 days is "%a %d" and 7 days
or more is "%Y/%m/%d".


xlabel
------

Sets the X axis label. The value is a strftime style format string.
Default depends on the graph duration: 24 hours or less is "Time
(%Z)", 24 hours to 7 days is "Day" and 7 days or more is "Date". The
datetime used to compute this is start, which may produce unexpected
results when a graph spans DST start or end.


dateformat
----------

Sets the format of the date labels at each end of X axis. The value is
a strftime style format string. Default is "%Y/%m/%d". The right hand
label is only drawn if it differs from the left. To have no labels,
set an empty format: "<dateformat></dateformat>"


xtics
-----

Sets the spacing of the "tic" marks on the X axis. The value is an
integer number of hours. The default is to allow gnuplot to set an
appropriate interval.


title
-----

Sets the title of the graph. A single line of text, for example:
"<title>Today's weather</title>". This title appears at the very top
of the graph, outside any plot area.


subplot
-------

Every plot element should contain at least one subplot element. A
separate trace is drawn for each subplot element, but all share the
same X and Y axes.


bmargin
-------

Sets the bottom margin, i.e. the spacing between the lower X axis and
the edge of the graph (or the next plot). The default is to let
gnuplot adjust this automatically, which works OK most of the time but
you may wish to fine tune the value to suit your installation.

The permitted value is any non-negative real number. On my setup 0.9
is a good value, set as follows: "<bmargin>0.9</bmargin>".


yrange
------

Sets the lower and upper limits of the (left hand) Y axis. The value
is anything understood by gnuplot, typically a pair of numbers. The
default is to allow gnuplot to set appropriate values, which is
unlikely to be what you want. For example, to plot typical UK
temperatures with no value going off the graph: "<yrange>-10,
30</yrange>". Note that commas are converted to colons, so
"<yrange>-10:30</yrange>" would be equivalent.

You can use an asterisk to have gnuplot choose a suitable value. For
example, to have the upper value auto scale whilst fixing the lower
value at zero, use "<yrange>0:*</yrange>".


y2range
-------

Sets the lower and upper limits of the right hand Y axis. Default is
for the right hand Y axis to be the same as the left, but setting a
different range is useful in dual axis plotting.


ytics
-----

Controls the "tic" marks on the left hand Y axis. The value can be
anything that's understood by gnuplot. For example, to set the tic
spacing to 45 use "<ytics>45</ytics>". More complex things are also
possible, e.g. to label a wind direction graph with compass points,
use "<y2tics>('N' 0, 'E' 90, 'S' 180, 'W' 270, 'N' 360)</y2tics>".


y2tics
------

Controls the "tic" marks on the right hand axis. The format is the
same as that for ytics. Default behaviour is to copy the left hand tic
marks, but without labels.


ylabel
------

Adds a label to the (left hand) Y axis. For example, when plotting
temperature: "<ylabel>°C</ylabel>". If you use ylabel you will
probably want to adjust lmargin.


ylabelangle
-----------

Adjust the angle of the (left hand) Y axis label, if your version of
gnuplot supports it. For example, to write the label horizontally:
"<ylabelangle>90</ylabelangle>".


y2label
-------

Adds a label to the right hand Y axis. For example, when plotting
humidity: "<y2label>%</y2label>". This is mostly used when plotting
dual axis graphs. If you use y2label you will probably want to adjust
rmargin.


y2labelangle
------------

Adjust the angle of the right hand Y axis label, if your version of
gnuplot supports it. For example, to write the label horizontally:
"<y2labelangle>90</y2labelangle>".


grid
----

Adds a grid to the plot. In most situations gnuplot's default grid is
suitable, so no value is needed: "<grid></grid>". More control is
possible using any of the options understood by gnuplot's set grid
command. For example, to have horizontal grid lines only:
"<grid>ytics</grid>".


source
------

Select the weather data to be plotted. Permitted values are
"<source>raw</source>", "<source>hourly</source>",
"<source>daily</source>" and "<source>monthly</source>". Default is
raw. Note that the different sources have different data dictionaries,
so this choice affects ycalc.


boxwidth
--------

Sets the width of the "boxes" used when drawing bar graphs. The value
is an integer expression yielding a number of seconds. Default depends
on source: raw is 240, hourly is 2800 and daily is 2800 * 24.


title
-----

Sets the title of the plot. A single line of text, for example:
"<title>Temperature (°C)</title>". This title appears within the plot
area, above any subplot titles.


command
-------

Execute any gnuplot command, just before the main "plot" command. This
option allows advanced users to have greater control over the graph
appearance. The value is any valid gnuplot command, typically
beginning with the word set. For example: "<command>set key tmargin
center horizontal width 1 noreverse enhanced autotitles box linetype
-1 linewidth 1</command>". (Don't ask me what this example does — I'm
not an advanced user).


xcalc
-----

Controls the X axis positioning of plotted data values. The default
value of data['idx'] is correct for most data, but there are some
exceptions. For example, when plotting bar charts of hourly rainfall,
it's nice to centre the bars on 30 minutes past the hour:
"<xcalc>data['idx'].replace(minute=30, second=0)</xcalc>".


ycalc
-----

Selects the data to be plotted. Any one line Python expression that
returns a single float value can be used. At its simplest this just
selects one value from the "data" dictionary, for example:
"<ycalc>data['temp_out']</ycalc>" plots the external temperature. More
complex expressions are possible, and some helper functions are
provided. For example: "<ycalc>dew_point(data['temp_out'],
data['hum_out'])</ycalc>" plots the external dew point, and
"<ycalc>data['wind_ave'] * 3.6 / 1.609344</ycalc>" plots the average
wind speed in miles per hour.

Cumulative plots are also possible. The result of each ycalc
computation is stored and made available to the next computation in
the variable last_ycalc. This can be used with any data, but is most
useful with rainfall: "<ycalc>data['rain'] + last_ycalc</ycalc>".


axes
----

Selects which Y axis the data is plotted against. Default is the left
hand axis, but the right hand axis can be chosen with:
"<axes>x1y2</axes>". This can be used in conjunction with y2range to
plot two unrelated quantities on one graph, for example temperature
and humidity.


style
-----

Sets the line style for the graph. Default is a smooth continuous
line, thickness 1. To select a bar graph use: "<style>box</style>". To
select points without a connecting line use: "<style>+</style>" or
"<style>x</style>". To select a line thickness 3 (for example) use:
"<style>line 3</style>". The thickness of points can be set in a
similar fashion. For complete control (for advanced users) a full
gnuplot style can be set: "<style>smooth unique lc 5 lw 3</style>".


colour
------

Sets the colour of the subplot line or boxes. Any integer value is
accepted. The mapping of colours to numbers is set by gnuplot. Default
value is the previous colour plus one.


title
-----

Sets the title of the subplot. A single line of text, for example:
"<title>Temperature (°C)</title>". This title appears within the plot
area, next to a short segment of the line colour used for the subplot.


Detailed API
============

Plot graphs of weather data according to an XML recipe

   usage: python RunModule.py Plot [options] data_dir temp_dir xml_file output_file
   options are:
    -h or --help    display this help
   data_dir is the root directory of the weather data
   temp_dir is a workspace for temporary files e.g. /tmp
   xml_file is the name of the source file that describes the plot
   output_file is the name of the image file to be created e.g. 24hrs.png

-[ Functions ]-

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-[ Classes ]-

+------------+--------------------------------------------------------------------------------------------+
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class class pywws.Plot.BasePlotter(params, raw_data, hourly_data, daily_data, monthly_data, work_dir)

   DoPlot(input_file, output_file)

   GetChildren(node, name)

   GetValue(node, name, default)

class class pywws.Plot.Record

class class pywws.Plot.GraphPlotter(params, raw_data, hourly_data, daily_data, monthly_data, work_dir)

   GetPlotList()

   GetDefaultRows()

   GetDefaultPlotSize()

   GetPreamble()

   PlotData(plot_no, plot, source)

pywws.Plot.main(argv=None)
