Examples

The most common use cases of PyCha are saving the chart to a file and embedding the chart in an application.

Ex1: Save Chart to File

The files for this examples are precip.py and barchart.py.

precip.py

Tainan = (
    ('Jan', 32.7),
    ('Feb', 9.5),
    ('Mar', 25.5),
    ('Apr', 13.7),
    ('May', 41.5),
    ('Jun', 782.2),
    ('Jul', 526.9),
    ('Aug', 84.5),
    ('Sep', 356.5),
    ('Oct', 33.0),
    ('Nov', 42.9),
    ('Dec', 1.1),
)

Paris = (
    ('Jan', 54),
    ('Feb', 46),
    ('Mar', 54),
    ('Apr', 47),
    ('May', 63),
    ('Jun', 58),
    ('Jul', 54),
    ('Aug', 52),
    ('Sep', 54),
    ('Oct', 56),
    ('Nov', 56),
    ('Dec', 56),
)

barchart.py

import sys

import cairo

import pycha.bar

import precip

def barChart(output, chartFactory):
    surface = cairo.ImageSurface(cairo.FORMAT_ARGB32, 500, 300)

    dataSet = (
        ('Tainan', [(i, l[1]) for i, l in enumerate(precip.Tainan)]),
        ('Paris', [(i, l[1]) for i, l in enumerate(precip.Paris)]),
        )

    options = {
        'axis': {
            'x': {
                'ticks': [dict(v=i, label=l[0]) for i, l in enumerate(precip.Tainan)],
                'label': 'Month',
                'rotate': 25,
            },
            'y': {
                'tickCount': 4,
                'rotate': 25,
                'label': 'Precipitation (mm)'
            }
        },
        'background': {
            'chartColor': '#d8e7ec',
            'baseColor': '#efebe7',
            'lineColor': '#444444'
        },
        'colorScheme': '#6eafc1',
        'legend': {
            'hide': False,
            'position': {'top': 5, 'left': 5},
        },
        'padding': {
            'left': 135,
            'bottom': 55,
        },
        'title': 'Monthly Precipitation'
    }
    chart = chartFactory(surface, options)

    chart.addDataset(dataSet)
    chart.render()

    surface.write_to_png(output)

if __name__ == '__main__':
    if len(sys.argv) > 1:
        output = sys.argv[1]
    else:
        output = 'barchart.png'
    barChart('v' + output, pycha.bar.VerticalBarChart)
    barChart('h' + output, pycha.bar.HorizontalBarChart)

Let’s say we want to graph the monthly precipitations of Tainan, Taiwan, and Paris, France. We can store the data in a Python file called precip.py so that we can refer to it as precip.Tainan and precip.Paris.

In barchart.py, we first import the necessary modules.

Our program supports a commandline argument for the file name:

import sys

PyCha depends on Cairo:

import cairo

We want a bar chart:

import pycha.bar

We need our data:

import precip

We next define a class called barChart that takes in the output filename and the chart type:

def barChart(output, chartFactory):

Our data needs to be formatted for PyCha:

dataSet = (
        ('Tainan', [(i, l[1]) for i, l in enumerate(precip.Tainan)]),
        ('Paris', [(i, l[1]) for i, l in enumerate(precip.Paris)]),
        )

We next set our options:

options = {
    'axis': {
        'x': {
            'ticks': [dict(v=i, label=l[0]) for i, l in enumerate(precip.Tainan)],
            'label': 'Month',
            'rotate': 25,
        },
        'y': {
            'tickCount': 4,
            'rotate': 25,
            'label': 'Precipitation (mm)'
        }
    },
    'background': {
        'chartColor': '#d8e7ec',
        'baseColor': '#efebe7',
        'lineColor': '#444444'
    },
    'colorScheme': '#6eafc1',
    'legend': {
        'hide': False,
        'position': {'top': 5, 'left': 5},
    },
    'padding': {
        'left': 135,
        'bottom': 55,
    },
    'title': 'Monthly Precipitation'
}

Notice that the axis:x:ticks option sets the labels for the x-axis ticks.

The legend is hidden by default, so we unhide it. The legend:position option is used to place the legend in the desired location.

We now create the chart and render it:

chart = chartFactory(surface, options)

chart.addDataset(dataSet)
chart.render()

The final step is to save the chart as a png image file:

surface.write_to_png(output)

We initiate the program with:

if __name__ == '__main__':
    if len(sys.argv) > 1:
        output = sys.argv[1]
    else:
        output = 'barchart.png'
    barChart('v' + output, pycha.bar.VerticalBarChart)
    barChart('h' + output, pycha.bar.HorizontalBarChart)

Running the program from the command line with:

python barchart.py

produces the beautiful charts shown below.

Vertical bar chart:

Horizontal bar chart:

Reason for all the rain in the summer in Tainan? Typhoons.

Ex2: Embed Chart in GTK App

Coming soon. Please refer to Chavier in the source tree for an example.