
pywws.WindRose
**************


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

This routine plots one or more "wind roses" (see Wikipedia for a
description). Like Plot.py almost everything is controlled by an XML
"recipe" / template file.

Before writing your own 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 wind rose template. It plots wind over the
last 24 hours.

   <?xml version="1.0" encoding="ISO-8859-1"?>
   <graph>
     <windrose>
       <ycalc>data['wind_ave']</ycalc>
     </windrose>
   </graph>

In this example, the root element graph has one windrose element which
contains nothing more than a ycalc element.

The complete element hierarchy is shown below.

   graph
       windrose
           xcalc
           ycalc
           threshold
           colour
           yrange
           points
           source
           title
       start
       stop
       duration
       layout
       size
       fileformat
       lmargin, rmargin, tmargin, bmargin
       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.


windrose
--------

A separate plot is drawn for each windrose element, but all share the
same time period.


start
-----

This element sets the date & time of the wind roses. It is used in the
constructor of a Python datetime object. For example, to start 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 wind roses. It is
used in the constructor of a Python datetime object. For example, to
end at 10 am (local time) on new year's day 2009: ``<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 duration of wind roses, 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 grid that is wider than
it is tall. 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 is a height of 600 pixels and a width that depends on the
layout. 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 plots. Default is
png. Any string recognised by your installation of gnuplot should do.
For example: ``<fileformat>gif</fileformat>`` will produce a GIF
image.


lmargin, rmargin, tmargin, bmargin
----------------------------------

Over-rides the automatically computed left, right, top or bottom
margin. Supply any positive real number, for example
``<lmargin>1.3</lmargin>``. Some experimentation may be necessary to
find the best values.


title
-----

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


xcalc
-----

Selects if data is included in the wind rose. The value should be a
valid Python logical expression. For example, to plot a rose for
afternoon winds only: ``<xcalc>data['idx'].hour &gt;= 12</xcalc>``.
This allows aggregation of afternoon wind data over several days.
Remember that data is indexed in UTC, so you need to use an expression
that takes account of your time zone. The default value is 'True'.


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['wind_ave']</ycalc>``. To convert to mph use:
``<ycalc>data['wind_ave'] * 3.6 / 1.609344</ycalc>``. You are unlikely
to want to use anything other than 'wind_ave' here.


threshold
---------

Sets the thresholds for each colour on the rose petals. Defaults are
based on the Wikipedia example. The values should be a correctly
ordered list of real numbers, for example: ``<threshold>0.5, 3.5, 7.5,
12.5, 18.5, 24.5, 31.5</threshold>`` approximates to the Beaufort
scale, if ycalc has been set to convert windspeeds to mph.


colour
------

Sets the colours of the threshold petal segments. Any sequence of
integer values is accepted. The mapping of colours to numbers is set
by gnuplot. Default value is 0, 1, 2, 3, etc.


yrange
------

Sets the upper limits of the axes. The rose shows what percentage of
the time the wind came from a particular direction. For example, if
you live somewhere with a very steady wind you might want to allow
higher percentages than normal: ``<yrange>91</yrange>``. Auto-scaling
is also possible, using an asterisk: ``<yrange>*</yrange>``


points
------

Sets the text of the compass points. The defaults are 'N', 'S', 'E' &
'W'. For graphs in another language you can over-ride this, for
example: ``<points>'No', 'Zu', 'Oo', 'We'</points>``. (The preferred
way to do this is to create a language file, see Localisation.py.)


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.


title
-----

Sets the title of the plot. A single line of text, for example:
``<title>Morning winds</title>``. This title appears within the plot
area, above the threshold colour key.


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

Plot a "wind rose".

usage: python WindRose.py [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 ]-

+------------+--------------------------------------------------------------------------------------------+
| ``main``([ |                                                                                            |
| argv])     |                                                                                            |
+------------+--------------------------------------------------------------------------------------------+

-[ Classes ]-

+------------+--------------------------------------------------------------------------------------------+
| ``RosePlot |                                                                                            |
| ter``(para |                                                                                            |
| ms,        |                                                                                            |
| raw_data,  |                                                                                            |
| hourly_dat |                                                                                            |
| a, ...)    |                                                                                            |
+------------+--------------------------------------------------------------------------------------------+

class class pywws.WindRose.RosePlotter(params, raw_data, hourly_data, daily_data, monthly_data, work_dir)

   GetPlotList()

   GetDefaultRows()

   GetDefaultPlotSize()

   GetPreamble()

   PlotData(plot_no, plot, source)

pywws.WindRose.main(argv=None)
