Fix ErrorBarCurveItem.get_closest_coordinates snap-to-bounds typo

The error-bar snapping logic in `get_closest_coordinates` compared
`abs(y - y)` and `abs(x - x)` (always 0) instead of the distance to the
current curve point `abs(y - yi)` / `abs(x - xi)`. As a result, the
snap-to-error-bound branches were dead code and the closest coordinate
returned was always the central curve point, never the upper/lower error
bound — even when the cursor was clearly closest to a bar end.

Replace the four self-subtractions with the correct distance to the
sampled point so cursor queries snap to the nearest error-bar bound as
documented.

Author: PierreRaybaut

plotpy/items/curve/errorbar.py

@@ -221,13 +221,13 @@ def get_closest_coordinates(self, x: float, y: float) -> tuple[float, float]:
         xi = self.dsamp(self._x)[i]
         yi = self.dsamp(self._y)[i]
         xmin, xmax, ymin, ymax = self.get_minmax_arrays()
-        if abs(y - y) > abs(y - ymin[i]):
+        if abs(y - yi) > abs(y - ymin[i]):
             yi = ymin[i]
-        elif abs(y - y) > abs(y - ymax[i]):
+        elif abs(y - yi) > abs(y - ymax[i]):
             yi = ymax[i]
-        if abs(x - x) > abs(x - xmin[i]):
+        if abs(x - xi) > abs(x - xmin[i]):
             xi = xmin[i]
-        elif abs(x - x) > abs(x - xmax[i]):
+        elif abs(x - xi) > abs(x - xmax[i]):
             xi = xmax[i]
         return xi, yi