Backport PR #24481: Fix floating-point drift in oscilloscope example

tacaswell · meeseeksmachine · commit 5a31a8edcc9e · 2022-11-18T02:43:19.000Z
diff --git a/examples/animation/strip_chart.py b/examples/animation/strip_chart.py
@@ -26,13 +26,16 @@ def __init__(self, ax, maxt=2, dt=0.02):
 
     def update(self, y):
         lastt = self.tdata[-1]
-        if lastt > self.tdata[0] + self.maxt:  # reset the arrays
+        if lastt >= self.tdata[0] + self.maxt:  # reset the arrays
             self.tdata = [self.tdata[-1]]
             self.ydata = [self.ydata[-1]]
             self.ax.set_xlim(self.tdata[0], self.tdata[0] + self.maxt)
             self.ax.figure.canvas.draw()
 
-        t = self.tdata[-1] + self.dt
+        # This slightly more complex calculation avoids floating-point issues
+        # from just repeatedly adding `self.dt` to the previous value.
+        t = self.tdata[0] + len(self.tdata) * self.dt
+
         self.tdata.append(t)
         self.ydata.append(y)
         self.line.set_data(self.tdata, self.ydata)
