PlotPanel/PlotFrame update canvas only after mouse click or window resize

[hydrocat]

Hey @newville first, thanks for this library!

Not sure if it matters, but I am running on Linux with wayland.
wxmplot version is 2025.1.6

I tried running demo.py with the self-updating plots and they don't seem to work.
The screen updates only after some sort of interaction happens, such as left mouse clicks and window resizing.
I also wrote a small app that has a button to trigger update_traces and verify the behavior. I post that below.
I dug through your code a bit and I found a comment in plotpanel.py

def oplot(...):
# ...code
        if not delay_draw:
            self.draw()
            # self.canvas.Refresh()
        conf.ntrace = conf.ntrace + 1
        return _lines

After uncommenting # self.canvas.Refresh(), my small app works as expected. But the demo still doesn't. I didn't get around to fixing it, but explicitly calling self.plotframe.panel.canvas.Refresh() makes it work but isn't ideal.

Here is the small app:

from numpy import ones, linspace, cos, random
import wx
from wxmplot import PlotPanel

class Frame(wx.Frame):
    def __init__(self):
        super().__init__(None, title="Counter", size=wx.Size(400,400))
        self.panel = wx.Panel(self)

        self.button = wx.Button(self.panel, label="0")
        self.button.Bind(wx.EVT_BUTTON, self.on_click)

        self.plot = PlotPanel(self.panel, (300, 400))
        self.plot.plot([0], [1], color='red', linewidth=1)

        sizer = wx.BoxSizer(wx.VERTICAL)
        sizer.Add(self.button, 0, wx.EXPAND | wx.ALL, 5)
        sizer.Add(self.plot, 1, wx.EXPAND | wx.ALL, 5)

        self.panel.SetSizer(sizer)

    def on_click(self, event):
        # Make up some data
        npts = 20
        x  = linspace(0, 40.0, npts)
        y  = 0.4 * cos(x/2.0) + random.normal(scale=0.03, size=npts)

        # Plot these new points
        self.plot.update_line(0, x,y, True)

        # Uncomment to actually refresh the panel contents.
        # self.plot.canvas.Refresh()
app = wx.App(False)
Frame().Show()
app.MainLoop()

[hydrocat]

I think I found the issue, but I might be wrong.
Under matplotlib's matplotlib/backends/backend_wxagg.py, there is this class definition:

class FigureCanvasWxAgg(FigureCanvasAgg, _FigureCanvasWxBase):
    def draw(self, drawDC=None):
        """
        Render the figure using agg.
        """
        FigureCanvasAgg.draw(self)
        self.bitmap = self._create_bitmap()
        self._isDrawn = True
        self.gui_repaint(drawDC=drawDC)
        # Add this to make our demo.py
        # self.Refresh()

So, I guess it is a mistake of matplotlib ?

[hydrocat]

This problem does stem from matplotlib
matplotlib/matplotlib#10417

[hydrocat]

I am following the breadcrumbs and I'll just leave it here:

It works just fine under XWayland:

GDK_BACKEND=x11 python demo.py 

[newville]

@hydrocat Thanks for all of that. I think I have not tested on any Wayland-only machines, hoping that Xwayland would be a good test. Maybe that was overly optimistic? OTOH, if "switch underlying windowing platform" means only a 1-line change to a plotting package, that's not too bad either.

So: do I understand from matplotlib/matplotlib#10417 correctly that your added self.Refresh() to the FigureCanvasWxAgg fixes the problem?

I believe that the wxAgg backend does not actually use the no-longer-very-useful wx.ClientDC anyway and uses wx.PaintDC throughout.

But I will also say that I have seen other things that have suggested that maybe we here in wxmplot should just have define our own matplotlib backend, and effectively monkey-patch matplotlib's backend.

I don't love that idea, but in fact neither backend_wx.py nor backend_wxagg.py are that large, so we could consider doing that.

[hydrocat]

The guys at matplotlib have asked me to push a PR with this little fix. I am not sure how they validate it internally but hopefully you'll only need to update matplotlib to fix it ;)

[newville]

@hydrocat Thanks - I agree that fixing this in matplotlib's FigureCanvasWx* is the better way to go.