Make cursor text precision actually correspond to pointing precision.

anntzer · anntzer · commit ebf2282130e5 · 2020-03-16T12:52:00.000+01:00
Currently, the cursor text (x=..., y=...) is typically displayed with
much, much more precision than the mouse cursor has (typically, one
pixel); IOW the last digits displayed are typically meaningless.

Make ScalarFormatter.format_data_short convert the inter-pixel distance
to data space and compute the corresponding number of significant digits
to use (and don't drop the final zeroes if any, as they are now, well,
significant).

(A similar change could be applied to other formatters, e.g.
LogFormatter.)
diff --git a/lib/matplotlib/tests/test_image.py b/lib/matplotlib/tests/test_image.py
@@ -351,7 +351,7 @@ def test_cursor_data():
 
 @pytest.mark.parametrize(
     "data, text_without_colorbar, text_with_colorbar", [
-        ([[10001, 10000]], "[1e+04]", "[10001]"),
+        ([[10001, 10000]], "[1e+04]", "[10001.000]"),
         ([[.123, .987]], "[0.123]", "[0.123]"),
 ])
 def test_format_cursor_data(data, text_without_colorbar, text_with_colorbar):
diff --git a/lib/matplotlib/tests/test_ticker.py b/lib/matplotlib/tests/test_ticker.py
@@ -512,8 +512,9 @@ class TestScalarFormatter:
         (True, (6, 6), (-1e5, 1e5), 6, False),
     ]
 
-    @pytest.mark.parametrize('unicode_minus, result',
-                             [(True, "\N{MINUS SIGN}1"), (False, "-1")])
+    @pytest.mark.parametrize(
+        'unicode_minus, result',
+        [(True, "\N{MINUS SIGN}1.000"), (False, "-1.000")])
     def test_unicode_minus(self, unicode_minus, result):
         mpl.rcParams['axes.unicode_minus'] = unicode_minus
         assert (
@@ -558,6 +559,16 @@ def test_scilimits(self, sci_type, scilimits, lim, orderOfMag, fewticks):
         tmp_form.set_locs(ax.yaxis.get_majorticklocs())
         assert orderOfMag == tmp_form.orderOfMagnitude
 
+    def test_cursor_precision(self):
+        fig, ax = plt.subplots()
+        ax.set_xlim(-1, 1)  # Pointing precision of 0.001.
+        fmt = ax.xaxis.get_major_formatter().format_data_short
+        assert fmt(0) == "0.000"
+        assert fmt(0.0123) == "0.012"
+        assert fmt(0.123) == "0.123"
+        assert fmt(1.23) == "1.230"
+        assert fmt(12.3) == "12.300"
+
 
 class FakeAxis:
     """Allow Formatter to be called without having a "full" plot set up."""
diff --git a/lib/matplotlib/ticker.py b/lib/matplotlib/ticker.py
@@ -587,11 +587,39 @@ def set_powerlimits(self, lims):
 
     def format_data_short(self, value):
         # docstring inherited
+        if isinstance(value, np.ma.MaskedArray) and value.mask:
+            return ""
+        if self.axis.__name__ in ["xaxis", "yaxis"]:
+            if self.axis.__name__ == "xaxis":
+                axis_trf = self.axis.axes.get_xaxis_transform()
+                axis_inv_trf = axis_trf.inverted()
+                screen_xy = axis_trf.transform((value, 0))
+                neighbor_values = axis_inv_trf.transform(
+                    screen_xy + [[-1, 0], [+1, 0]])[:, 0]
+            else:  # yaxis:
+                axis_trf = self.axis.axes.get_yaxis_transform()
+                axis_inv_trf = axis_trf.inverted()
+                screen_xy = axis_trf.transform((0, value))
+                neighbor_values = axis_inv_trf.transform(
+                    screen_xy + [[0, -1], [0, +1]])[:, 1]
+            delta = abs(neighbor_values - value).max()
+        else:
+            # Rough approximation: no more than 1e4 pixels.
+            delta = self.axis.get_view_interval() / 1e4
+        # If e.g. value = 45.67 and delta = 0.02, then we want to round to
+        # 2 digits after the decimal point (floor(log10(0.02)) = -2);
+        # 45.67 contributes 2 digits before the decimal point
+        # (floor(log10(45.67)) + 1 = 2): the total is 4 significant digits.
+        # A value of 0 contributes 1 "digit" before the decimal point.
+        sig_digits = max(
+            0,
+            (math.floor(math.log10(abs(value))) + 1 if value else 1)
+            - math.floor(math.log10(delta)))
+        fmt = f"%-#.{sig_digits}g"
         return (
-            "" if isinstance(value, np.ma.MaskedArray) and value.mask else
             self.fix_minus(
-                locale.format_string("%-12g", (value,)) if self._useLocale else
-                "%-12g" % value))
+                locale.format_string(fmt, (value,)) if self._useLocale else
+                fmt % value))
 
     def format_data(self, value):
         # docstring inherited
