Add support for overriding x tick with non arithmetic progression values

CHANGELOG.md

@@ -6,6 +6,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Added
 - Support `marginal_x`/`marginal_y="heatmap"` in `density_heatmap`, drawing a single-row/column heatmap strip in the margin colored by the same `z`/`histfunc` aggregate as the main plot and sharing its color scale [[#5706](https://github.com/plotly/plotly.py/issues/5706)], with thanks to @lucasjamar for the contribution!
+- Add support for custom tick values in `mpl_to_plotly` when the matplotlib tick positions don't follow an arithmetic progression, including custom tick labels and tick values on date axes [[#5262](https://github.com/plotly/plotly.py/pull/5262)], with thanks to @robertoffmoura for the contribution!
 
 ### Fixed
 - Fix `mpl_to_plotly` not setting `paper_bgcolor` and `plot_bgcolor` from the matplotlib figure and axes backgrounds, so converted figures match the source figure's background colors [[#5285](https://github.com/plotly/plotly.py/pull/5285)], with thanks to @robertoffmoura for the contribution!

plotly/matplotlylib/mpltools.py

@@ -436,18 +436,20 @@ def prep_ticks(ax, index, ax_type, props):
             tick0 = tickvalues[0]
             dticks = [
                 round(tickvalues[i] - tickvalues[i - 1], 12)
-                for i in range(1, len(tickvalues) - 1)
+                for i in range(1, len(tickvalues))
             ]
-            if all([dticks[i] == dticks[i - 1] for i in range(1, len(dticks) - 1)]):
+            if all([dticks[i] == dticks[i - 1] for i in range(1, len(dticks))]):
                 dtick = tickvalues[1] - tickvalues[0]
             else:
                 warnings.warn(
                     "'linear' {0}-axis tick spacing not even, "
-                    "ignoring mpl tick formatting.".format(ax_type)
+                    "exporting explicit tick values instead of dtick.".format(ax_type)
                 )
                 raise TypeError
         except (IndexError, TypeError):
             axis_dict["nticks"] = props["axes"][index]["nticks"]
+            if props["axes"][index]["tickvalues"] is not None:
+                axis_dict["tickvals"] = props["axes"][index]["tickvalues"]
         else:
             axis_dict["tick0"] = tick0
             axis_dict["dtick"] = dtick
@@ -485,17 +487,26 @@ def prep_ticks(ax, index, ax_type, props):
     formatter = axis.get_major_formatter().__class__.__name__
     if ax_type == "x" and "DateFormatter" in formatter:
         axis_dict["type"] = "date"
-        try:
-            axis_dict["tick0"] = mpl_dates_to_datestrings(axis_dict["tick0"], formatter)
-        except KeyError:
-            pass
-        finally:
+        tickvalues = props["axes"][index]["tickvalues"]
+        if tickvalues is not None:
+            # custom ticks: export the exact locations and drop the
+            # arithmetic tick0/dtick spec, which plotly would use instead
+            axis_dict["tickvals"] = mpl_dates_to_datestrings(tickvalues, formatter)
+            axis_dict.pop("tick0", None)
             axis_dict.pop("dtick", None)
             axis_dict.pop("tickmode", None)
-            axis_dict["range"] = mpl_dates_to_datestrings(props["xlim"], formatter)
+        axis_dict["range"] = mpl_dates_to_datestrings(props["xlim"], formatter)
 
     if formatter == "LogFormatterMathtext":
         axis_dict["exponentformat"] = "e"
+    elif (
+        formatter in ("FuncFormatter", "FixedFormatter")
+        and props["axes"][index]["tickformat"] is not None
+    ):
+        axis_dict.pop("dtick", None)
+        axis_dict.pop("tickmode", None)
+        axis_dict["ticktext"] = props["axes"][index]["tickformat"]
+        axis_dict["tickvals"] = props["axes"][index]["tickvalues"]
     return axis_dict
 
 

plotly/matplotlylib/tests/test_renderer.py

@@ -233,3 +233,110 @@ def test_semitransparent_axes_background_preserved():
     plotly_fig = tls.mpl_to_plotly(fig)
 
     assert plotly_fig.layout.plot_bgcolor == "rgba(26, 51, 76, 0.4)"
+
+
+def test_non_arithmetic_progression_xtickvals():
+    xticks = [0.01, 0.53, 0.75]
+    fig, ax = plt.subplots()
+    ax.plot([0, 1], [0, 1])
+    ax.set_xticks(xticks)
+
+    plotly_fig = tls.mpl_to_plotly(fig)
+
+    assert plotly_fig.layout.xaxis.tickvals == tuple(xticks)
+
+
+def test_non_arithmetic_progression_yticks():
+    yticks = [0.01, 0.53, 0.75]
+    fig, ax = plt.subplots()
+    ax.plot([0, 1], [0, 1])
+    ax.set_yticks(yticks)
+
+    plotly_fig = tls.mpl_to_plotly(fig)
+
+    assert plotly_fig.layout.yaxis.tickvals == tuple(yticks)
+
+
+def test_non_arithmetic_progression_xticktext():
+    xtickvals = [0.01, 0.53, 0.75]
+    xticktext = ["Baseline", "param = 1", "param = 2"]
+    fig, ax = plt.subplots()
+    ax.plot([0, 1], [0, 1])
+    ax.set_xticks(xtickvals, xticktext)
+
+    plotly_fig = tls.mpl_to_plotly(fig)
+
+    assert plotly_fig.layout.xaxis.tickvals == tuple(xtickvals)
+    assert plotly_fig.layout.xaxis.ticktext == tuple(xticktext)
+
+
+def test_fixed_formatter_ticktext():
+    import matplotlib.ticker as ticker
+
+    fig, ax = plt.subplots()
+    ax.plot([0, 1], [0, 1])
+    ax.xaxis.set_major_locator(ticker.FixedLocator([0.01, 0.53, 0.75]))
+    ax.xaxis.set_major_formatter(
+        ticker.FixedFormatter(["Baseline", "param = 1", "param = 2"])
+    )
+
+    plotly_fig = tls.mpl_to_plotly(fig)
+
+    assert plotly_fig.layout.xaxis.tickvals == (0.01, 0.53, 0.75)
+    assert plotly_fig.layout.xaxis.ticktext == ("Baseline", "param = 1", "param = 2")
+
+
+def test_custom_date_xtickvals_are_converted():
+    """Custom tick values on a date axis must be converted to date strings,
+    not left as raw matplotlib date numbers or datetime objects."""
+    dates = [datetime.datetime(2023, 1, i) for i in range(1, 11)]
+    fig, ax = plt.subplots()
+    ax.plot(dates, np.random.rand(10))
+    ax.set_xticks(dates[::3])
+
+    plotly_fig = tls.mpl_to_plotly(fig)
+
+    assert plotly_fig.layout.xaxis.tickvals == (
+        "2023-01-01 00:00:00",
+        "2023-01-04 00:00:00",
+        "2023-01-07 00:00:00",
+        "2023-01-10 00:00:00",
+    )
+
+
+def test_uneven_custom_date_xtickvals_are_converted():
+    """Unevenly spaced custom date ticks must be converted to date strings."""
+    dates = [datetime.datetime(2023, 1, i) for i in range(1, 11)]
+    ticks = [datetime.datetime(2023, 1, i) for i in [1, 3, 6, 10]]
+    fig, ax = plt.subplots()
+    ax.plot(dates, np.random.rand(10))
+    ax.set_xticks(ticks)
+
+    plotly_fig = tls.mpl_to_plotly(fig)
+
+    assert plotly_fig.layout.xaxis.tickvals == (
+        "2023-01-01 00:00:00",
+        "2023-01-03 00:00:00",
+        "2023-01-06 00:00:00",
+        "2023-01-10 00:00:00",
+    )
+
+
+def test_custom_date_xtickvals_given_as_numbers_are_converted():
+    """Custom date ticks given as matplotlib date numbers must be converted
+    to date strings."""
+    import matplotlib.dates as mdates
+
+    dates = [datetime.datetime(2023, 1, i) for i in range(1, 11)]
+    fig, ax = plt.subplots()
+    ax.plot(dates, np.random.rand(10))
+    ax.set_xticks([mdates.date2num(d) for d in dates[::3]])
+
+    plotly_fig = tls.mpl_to_plotly(fig)
+
+    assert plotly_fig.layout.xaxis.tickvals == (
+        "2023-01-01 00:00:00",
+        "2023-01-04 00:00:00",
+        "2023-01-07 00:00:00",
+        "2023-01-10 00:00:00",
+    )