Matplotlib-style transforms on PyQtGraph PlotItems

[Brad Buran]

I'm updating some code that was written in Chaco to use PyQtGraph instead. I'm working with relatively low-level primitives from PyQtGraph to maximize my control over the various components. For example, I am creating a GraphicsLayout, inserting a ViewBox and then adding the PlotCurveItem to the ViewBox.

One thing I would like to accomplish in a single ViewBox is to have multiple PlotCurveItems. However, each curve needs to be offset vertically relative to the axes coordinates. For example, if I have three plots, the plots would be spaced equally along the vertical axis. However, when I use the mouse wheel to scale the Y-axis, it should scale the plot (but not the position of the plot).

This is easy to accomplish in Matplotlib using their transform system. For example, I would do:

import matplotlib as mp
import pylab as pl
import numpy as np

ax = pl.gca()

x = np.arange(100)
y = np.random.uniform(-1, 1, size=100)

offset_transform = mp.transforms.Affine2D().translate(0, 0.25)
new_transform = ax.transLimits + offset_transform + ax.transAxes
ax.plot(x, y, 'ro-', transform=new_transform)

offset_transform = mp.transforms.Affine2D().translate(0, 0.5)
new_transform = ax.transLimits + offset_transform + ax.transAxes
ax.plot(x, y, 'go-', transform=new_transform)

offset_transform = mp.transforms.Affine2D().translate(0, 0.75)
new_transform = ax.transLimits + offset_transform + ax.transAxes
ax.plot(x, y, 'bo-', transform=new_transform)

#ax.axis(ymin=0, ymax=5)
pl.show()

Does PyQtGraph have a similar system?

[Luke Campagnola]

PyQtGraph's coordinate system is inherited from Qt's GraphicsView (http://doc.qt.io/qt-4.8/graphicsview.html). Basically, it uses a scenegraph where each item in the scene is assigned a 2D matrix transform, and the final transform used to draw each item is generated on-demand by multiplying the item's transform with the transforms of its parents. This system is efficient but not very flexible (and it's maybe the thing I dislike the most about using Qt GraphicsView).

So the PyQtGraph equivalent just involves manually updating the position of each curve whenever the view range has changed:

import pyqtgraph as pg

import numpy as np

plt = pg.plot()

curves = [plt.plot(np.random.normal(size=100)) for i in range(5)]

# Don't try to scale the y-axis automatically

plt.enableAutoRange(x=True, y=False)

plt.setYRange(-30, 30)

def updateCurves():

    # Set a good y position for each curve 

    vb = plt.plotItem.vb

    height = vb.height()

    for i,curve in enumerate(curves):

        y = vb.mapToView(pg.Point(0, (i+1) * height / (len(curves)+1))).y()

        curve.setPos(0, y)

plt.plotItem.vb.sigRangeChanged.connect(updateCurves)

updateCurves()

Luke

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