[Q&A] Real-time plot support in WPF

[Legosharkie]

Your question

Are real-time plots supported in WPF?

I have looked all over the wiki, samples and cookbook but I can't figure out how to update plots like in Avalonia. I tried to do the example in the cookbook with the StreamingSignal, I added the binding to a Figure to my MplChartControl in my .xaml and then I subscribe to my "sf.RenderRequested", where sf = StreamingFigure, but I cannot find how I tell the UI to render my StreamingFigure without explicitly setting

Figure = null; 
Figure = sf.Figure();

inside my Dispatcher.Invoke.

Does WPF only support "static" plots?

MatPlotLibNet version

1.18.0

Code sample (optional)

No response

[xkqg]

You found a real gap, and it is ours rather than anything you did wrong.

Three of the four native controls ship a streaming counterpart next to the ordinary one — MplStreamingChartControl for Avalonia, MplStreamingChartView for MAUI, MplStreamingChartElement for Uno. Each takes a StreamingFigure, subscribes to RenderRequested itself, and marshals a redraw to the UI thread. WPF never got that one, so there was nothing in the wiki or the cookbook for you to find.

That also explains the trick you had to invent. MplChartControl.Figure is a dependency property, and WPF raises its changed-callback only when the reference changes — setting the same figure again does nothing, so null and back is the only way to fake a new one.

You do not need it, though. The control re-reads Figure on every paint, so telling it to paint is enough:

chart.Figure = sf.Figure;                       // once

sf.RenderRequested += () => Dispatcher.Invoke(() =>
{
    sf.ApplyAxisScaling();                      // let the axes follow the data
    chart.InvalidateVisual();                   // "paint again"
});

The ApplyAxisScaling() is worth keeping even with your current approach: without it the axes stay where they started while the data moves past them. It is the same pair of calls the Avalonia control makes internally.

We are adding the missing WPF control, so this becomes one binding like everywhere else. It goes into MatPlotLibNet.Wpf itself — no new package — in the next patch release. Thanks for the careful write-up; it said exactly where you looked, which is what made the gap obvious.

[xkqg]

1.18.1 is being published now, and MplStreamingChartControl is in it.

<wpf:MplStreamingChartControl StreamingFigure="{Binding LiveChart}" />

Bind the figure once and leave it there — no null and back, and no handler of your own. The control subscribes to the figure, marshals the redraw to the dispatcher, scales the axes before each draw, and lets go when it is unloaded.

Tracing your scenario end to end turned up a second thing on the way, in the core package: StreamingFigure.MinRenderInterval had a setter, but the clock behind it was started once when the figure was built and never restarted. Asking for a calmer chart — directly or through BuildStreaming(renderInterval) — quietly kept giving you thirty-three milliseconds, and the property handed your own value back so nothing looked wrong. Measured before the fix: half a second asked for, twenty appends over two hundred milliseconds, ten redraws delivered where half a second allows one. It now does what it says, so this works too:

sf.MinRenderInterval = TimeSpan.FromMilliseconds(100);   // ten frames a second

The wiki page on streaming lists WPF among the hosts now, and the cookbook's streaming page shows the binding for each of them. Thanks again — the report was precise enough to find both.