Make cursor text precision actually correspond to pointing precision.

anntzer · anntzer · commit bfb9cc798d71 · 2020-03-17T13:05:03.000+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/doc/users/next_whats_new/2020-03-15-cursor-sigdigits.rst b/doc/users/next_whats_new/2020-03-15-cursor-sigdigits.rst
@@ -0,0 +1,6 @@
+Cursor text now uses a number of significant digits matching pointing precision
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Previously, the x/y position displayed by the cursor text would usually include
+far more significant digits than the mouse pointing precision (typically one
+pixel).  This is now fixed for linear scales.
diff --git a/lib/matplotlib/tests/test_ticker.py b/lib/matplotlib/tests/test_ticker.py
@@ -558,6 +558,16 @@ 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
@@ -168,6 +168,7 @@
 import logging
 import locale
 import math
+from numbers import Integral
 
 import numpy as np
 
@@ -587,11 +588,42 @@ def set_powerlimits(self, lims):
 
     def format_data_short(self, value):
         # docstring inherited
+        if isinstance(value, np.ma.MaskedArray) and value.mask:
+            return ""
+        if isinstance(value, Integral):
+            fmt = "%d"
+        else:
+            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
