Fix plot legend don't show up when dataset start with zero (#10176)

* added test to plot_histogram with dataset starting with zero

* fixed plot_histogram

* added docs

---------

Co-authored-by: Matthew Treinish <mtreinish@kortar.org>
(cherry picked from commit 645598a)

qiskit/visualization/counts_visualization.py

@@ -343,8 +343,8 @@ def _plotting_core(
     labels_dict, all_pvalues, all_inds = _plot_data(data, labels, number_to_keep, kind=kind)
     rects = []
     for item, _ in enumerate(data):
+        label = None
         for idx, val in enumerate(all_pvalues[item]):
-            label = None
             if not idx and legend:
                 label = legend[item]
             if val > 0:
@@ -358,6 +358,7 @@ def _plotting_core(
                         zorder=2,
                     )
                 )
+                label = None
         bar_center = (width / 2) * (length - 1)
         ax.set_xticks(all_inds[item] + bar_center)
         ax.set_xticklabels(labels_dict.keys(), fontsize=14, rotation=70)

releasenotes/notes/fix-plot-legend-not-showing-up-3202bec143529e49.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Fixed plot legend error when your dataset has a zero value at first position.
+    When one of your counts or distributions had a zero value at first position, the relative legend didn't show up.
+    See `#10158 <https://github.com/Qiskit/qiskit-terra/issues/10158>` for more details.

test/python/visualization/test_plot_histogram.py

@@ -214,6 +214,19 @@ def test_with_number_to_keep_multiple_executions_correct_image(self):
         mpl.pyplot.close(figure_ref)
         mpl.pyplot.close(figure_truncated)
 
+    @unittest.skipUnless(optionals.HAS_MATPLOTLIB, "matplotlib not available.")
+    def test_number_of_items_in_legend_with_data_starting_with_zero(self):
+        """Test legend if there's a 0 value at the first item of the dataset"""
+        dist_1 = {"0": 0.369, "1": 0.13975}
+        dist_2 = {"0": 0, "1": 0.48784}
+        legend = ["lengend_1", "lengend_2"]
+        plot = plot_histogram([dist_1, dist_2], legend=legend)
+        self.assertEqual(
+            len(plot._localaxes[0].legend_.texts),
+            2,
+            "Plot should have the same number of legend items as defined",
+        )
+
 
 if __name__ == "__main__":
     unittest.main(verbosity=2)