Custom graphical devices

Warning

This is still very experimental, and using this may result in crashing the Python interpreter.

The C-API to R allows extension writers to implement custom graphical devices (using C). This feature was used to implement drivers to SVG or Cairo, for example (Cairo support made it later to the R codebase).

Rpy2 is exposing the creation of custome graphical devies to Python programmer, without the need for C.

To demonstrate how to implement a graphical, we consider the following example: a device that counts the number of times graphical primitives are used. This is something of very limited practical use, but enough to explain the principles.

Such a device would be implemented as follows:

import rpy2.rinterface._rpy_device as rdevice
from collections import Counter

class BeancounterDevice(rdevice.GraphicalDevice):
    """ Graphical devive for R that counts the
    number of times primitives are called."""

    def __init__(self):
        super(BeancounterDevice, self).__init__()
        self._ct = Counter()

    def circle(self, x, y, radius):
        self._ct['circle'] += 1

    def clip(self, x0, x1, y0, y1):
        self._ct['clip'] += 1

    def line(self, x1, y1, x2, y2):
        self._ct['lines'] += 1

    def mode(self, mode):
        self._ct['mode'] += 1

    def rect(self, x0, x1, y0, y1):
        self._ct['rectangle'] += 1

    def strwidth(self, text):
        self._ct['strwidth'] += 1
        return float(0)

    def text(x, y, string, rot, hadj):
        self._ct['text'] += 1

The class BeancounterDevice can now be used as genuine R plotting device.

from rpy2.robjects.packages import importr

dev = BeancounterDevice()

graphics = importr("graphics")
# plot into our counting device
graphics.plot(0, 0)

# Print the counts
print(dev._ct)

To implement a new custom graphical device for R, one only has to extend the class rpy2.rinterface._rpy_device.GraphicalDevice. Error messages will be printed if that new device does not implement functionalities used by R.

The Python documentation strings for the class and its methods are: