Matplotlib show parasite axes using OO API

Question

I am totally stumped at how to use parasite axes using the OO API (Figure embedded in PyQt4). I want to plot an axis offset from the plot as per the code below. However when embedding matplotlib in PyQt4 one can't use pyplot or host_subplot.

from mpl_toolkits.axes_grid1 import host_subplot
import mpl_toolkits.axisartist as AA
import matplotlib.pyplot as plt

host = host_subplot(111, axes_class=AA.Axes)
plt.subplots_adjust(right=0.75)

par1 = host.twinx()
par2 = host.twinx()

offset = 60
new_fixed_axis = par2.get_grid_helper().new_fixed_axis
par2.axis["right"] = new_fixed_axis(loc="right",
                                axes=par2,
                                offset=(offset, 0))

par2.axis["right"].toggle(all=True)

host.set_xlim(0, 2)
host.set_ylim(0, 2)

par1.set_ylabel("Temperature")
par2.set_ylabel("Velocity")

p2, = par1.plot([0, 1, 2], [0, 3, 2], label="Temperature")
p3, = par2.plot([0, 1, 2], [50, 30, 15], label="Velocity")

par1.set_ylim(0, 4)
par2.set_ylim(1, 65)

plt.draw()
plt.show()

I did manage to plot an offset axis by creating two Figures and linking their axes as per the question here:

Matplotlib show one axis and no data

This is my preferred method as it allows me to put a QSplitter between the figures, however I cannot get the upper plot to show the X axis only - some of the graph plot and its data always show no matter how small I set the y axis to be.

I then tried drawing my own offset axis in a separate QWidget by obtaining the plots axis geometry:

ax = foo.figure.add_subplot(1,1,1)
lineGeometry = [ax.bbox.x0, etc.]

then using the axis geometry to draw a line with Qt.QPainter() the same start/end position but offset from the plot in a separate QWidget. This is a bit clunky and I would prefer a simpler way.

Any help with using parasitic axes embedded in Qt4 would be appreciated.

Don Smythe Don Smythe · Accepted Answer · 2015-02-22T12:29:58

Thanks to comments at my previous post noted above I managed to work out that you can use matplotlib.pyplot within PyQt4, but use the qt4 backend for display. A long winded (apologies) example is below. There is a bug with the code though both plots appear to be plotting on top of each other, not sure whats going on there:

import sys
from PyQt4 import QtGui, QtCore
import numpy as np
import matplotlib
from matplotlib.patches import Circle
from matplotlib.collections import PatchCollection
import matplotlib.pyplot as plt
from matplotlib.backends.backend_qt4agg import FigureCanvasQTAgg as     FigureCanvas
from mpl_toolkits.axes_grid1 import host_subplot
import mpl_toolkits.axisartist as AA
from matplotlib.figure import Figure

class ApplicationWindow(QtGui.QMainWindow):
    def __init__(self):
        QtGui.QMainWindow.__init__(self)
        self.setAttribute(QtCore.Qt.WA_DeleteOnClose)
        self.main_widget = QtGui.QWidget(self)
        l = QtGui.QVBoxLayout(self.main_widget)
        dc = PyPlotCanvas(self.main_widget, width=5, height=4, dpi=100)
        ac = AxisArtistOO(self.main_widget, width=5, height=4, dpi=100)
        l.addWidget(dc)
        l.addWidget(ac)
        self.main_widget.setFocus()
        self.setCentralWidget(self.main_widget)

class PyPlotCanvas(FigureCanvas):
    """Matplotlib pyplot as FigureCanvas"""
    def __init__(self, parent=None, width=5, height=4, dpi=100):
        self.fig = Figure(figsize=(width, height), dpi=dpi)
        self.axes = self.fig.add_subplot(111)
        self.compute_initial_figure()
        FigureCanvas.__init__(self, self.fig)
        self.setParent(parent)
        FigureCanvas.setSizePolicy(self,
                               QtGui.QSizePolicy.Expanding,
                               QtGui.QSizePolicy.Expanding)
        FigureCanvas.updateGeometry(self)

    def compute_initial_figure(self):
        fig, ax = plt.subplots()
        N = 3
        x       = np.random.rand(N)
        y       = np.random.rand(N)
        radii   = 0.1*np.random.rand(N)
        patches = []
        for x1,y1,r in zip(x, y, radii):
            circle = Circle((x1,y1), r)
            patches.append(circle)
        colors = 100*np.random.rand(len(patches))
        p = PatchCollection(patches, cmap=matplotlib.cm.jet, alpha=0.4)
        p.set_array(np.array(colors))
        ax.add_collection(p)
        plt.colorbar(p)
        self.fig = fig

class AxisArtistOO(FigureCanvas):
    def __init__(self, parent=None, width=5, height=4, dpi=100):
        self.fig = Figure(figsize=(width, height), dpi=dpi)
        self.axes = self.fig.add_subplot(111)
        self.compute_initial_figure()
        FigureCanvas.__init__(self, self.fig)
        self.setParent(parent)
        FigureCanvas.setSizePolicy(self,
                               QtGui.QSizePolicy.Expanding,
                               QtGui.QSizePolicy.Expanding)
        FigureCanvas.updateGeometry(self)

    def compute_initial_figure(self):
        host = host_subplot(111, axes_class=AA.Axes)
        plt.subplots_adjust(right=0.75)
        par1 = host.twinx()
        par2 = host.twinx()
        offset = 60
        new_fixed_axis = par2.get_grid_helper().new_fixed_axis
        par2.axis["right"] = new_fixed_axis(loc="right",
                                    axes=par2,
                                    offset=(offset, 0))
        par2.axis["right"].toggle(all=True)
        host.set_xlim(0, 2)
        host.set_ylim(0, 2)
        par1.set_ylabel("Temperature")
        par2.set_ylabel("Velocity")
        p2, = par1.plot([0, 1, 2], [0, 3, 2], label="Temperature")
        p3, = par2.plot([0, 1, 2], [50, 30, 15], label="Velocity")
        par1.set_ylim(0, 4)
        par2.set_ylim(1, 65)
        fig, ax = plt.subplots()
        self.fig = fig

qApp = QtGui.QApplication(sys.argv)
aw = ApplicationWindow()
aw.show()
sys.exit(qApp.exec_())

Matplotlib show parasite axes using OO API

1 Answers