ensure histogram bin step size is always a positive integer

Author: p-j-smith

Diff for: src/napari_matplotlib/histogram.py

@@ -72,7 +72,7 @@ def draw(self) -> None:
         # whole cube into memory.
         if data.dtype.kind in {"i", "u"}:
             # Make sure integer data types have integer sized bins
-            step = 1 + (np.max(data) - np.min(data)) // 100
+            step = 1 + abs(np.max(data) - np.min(data)) // 100
             bins = np.arange(np.min(data), np.max(data) + step, step)
         else:
             bins = np.linspace(np.min(data), np.max(data), 100)