Efficiently Plotting Many Lines in VisPy

Question

From all example code/demos I have seen in the VisPy library, I only see one way that people plot many lines, for example:

for i in range(N):
    pos = pos.copy()
    pos[:, 1] = np.random.normal(scale=5, loc=(i+1)*30, size=N)
    line = scene.visuals.Line(pos=pos, color=color, parent=canvas.scene)
    lines.append(line)
canvas.show()

My issue is that I have many lines to plot (each several hundred thousand points). Matplotlib proved too slow because of the total number of points plotted was in the millions, hence I switched to VisPy. But VisPy is even slower when you plot thousands of lines each with thousands of points (the speed-up comes when you have millions of points).

The root cause is in the way lines are drawn. When you create a plot widget and then plot a line, each line is rendered to the canvas. In matplotlib you can explicitly state to not show the canvas until all lines are drawn in memory, but there doesn't appear to be the same functionality in VisPy, making it useless.

Is there any way around this? I need to plot multiple lines so that I can change properties interactively, so flattening all the data points into one plot call won't work.

(I am using a PyQt4 to embed the plot in a GUI. I have also considered pyqtgraph.)

Answer 1

You should pass an array to the "connect" parameter of the Line() function.

    xy = np.random.rand(5,2) # 2D positions
    # Create an array of point connections :
    toconnect = np.array([[0,1], [0,2], [1,4], [2,3], [2,4]])
    # Point 0 in your xy will be connected with 1 and 2, point 
    # 1 with 4 and point 2 with 3 and 4.
    line = scene.visuals.Line(pos=xy, connect=toconnect)

You only add one object to your canvas but the control pear line is more limited.