add legend as dict for line plots with test

RELEASE_NOTES.md

@@ -2,6 +2,7 @@
 # Next Release
 
 - (#73)[https://github.com/IAMconsortium/pyam/pull/73] Adds ability to remove labels for markers, colors, or linestyles
+- (#71)[https://github.com/IAMconsortium/pyam/pull/71] Line plots `legend` keyword can now be a dictionary of legend arguments
 - (#70)[https://github.com/IAMconsortium/pyam/pull/70] Support reading of both SSP and RCP data files downloaded from the IIASA database.
 - (#66)[https://github.com/IAMconsortium/pyam/pull/66] Fixes a bug in the `interpolate()` function (duplication of data points if already defined)
 - (#65)[https://github.com/IAMconsortium/pyam/pull/65] Add a `filter_by_meta()` function to filter/join a pd.DataFrame with an IamDataFrame.meta table

pyam/plotting.py

@@ -26,6 +26,7 @@
 except ImportError:
     from functools32 import lru_cache
 
+from pyam.logger import logger
 from pyam.run_control import run_control
 from pyam.utils import requires_package, SORT_IDX, isstr
 
@@ -35,6 +36,9 @@
 # explicitly declared
 _DEFAULT_PROPS = None
 
+# maximum number of labels after which do not show legends by default
+MAX_LEGEND_LABELS = 13
+
 
 def reset_default_props(**kwargs):
     """Reset properties to initial cycle point"""
@@ -205,13 +209,13 @@ def region_plot(df, column='value', ax=None, crs=None, gdf=None, add_features=Tr
             )
         cb = plt.colorbar(scalar_map, **cbar)
 
-    if legend:
+    if legend is not False:
         if legend is True:  # use some defaults
             legend = dict(
                 bbox_to_anchor=(1.32, 0.5) if cbar else (1.2, 0.5),
                 loc='right',
             )
-        ax.legend(handles, labels, **legend)
+        _add_legend(ax, handles, labels, legend)
 
     if title:
         var = df['variable'].unique()[0]
@@ -471,9 +475,10 @@ def line_plot(df, x='year', y='value', ax=None, legend=None, title=True,
         The column to use for y-axis values
         default: value
     ax : matplotlib.Axes, optional
-    legend : bool, optional
-        Include a legend (`None` displays legend only if less than 13 entries)
-        default: None
+    legend : bool or dictionary, optional
+        Add a legend. If a dictionary is provided, it will be used as keyword
+        arguments in creating the legend.
+        default: None (displays legend only if less than 13 entries)
     title : bool or string, optional
         Display a default or custom title.
     color : string, optional
@@ -496,7 +501,6 @@ def line_plot(df, x='year', y='value', ax=None, legend=None, title=True,
         default: []
     kwargs : Additional arguments to pass to the pd.DataFrame.plot() function
     """
-
     if ax is None:
         fig, ax = plt.subplots()
 
@@ -524,11 +528,11 @@ def line_plot(df, x='year', y='value', ax=None, legend=None, title=True,
             prop_idx[kind] = df.columns.names.index(var)
 
     # plot data, keeping track of which legend labels to apply
-    legend_data = []
     no_label = [rm_legend_label] if isstr(rm_legend_label) else rm_legend_label
     for col, data in df.iteritems():
         pargs = {}
         labels = []
+        # build plotting args and line legend labels
         for key, kind, var in [('c', 'color', color),
                                ('marker', 'marker', marker),
                                ('linestyle', 'linestyle', linestyle)]:
@@ -539,19 +543,18 @@ def line_plot(df, x='year', y='value', ax=None, legend=None, title=True,
                     labels.append(repr(label).lstrip("u'").strip("'"))
             else:
                 pargs[key] = var
-
-        legend_data.append(' '.join(labels))
         kwargs.update(pargs)
         data.plot(ax=ax, **kwargs)
+        if labels:
+            ax.lines[-1].set_label(' '.join(labels))
 
-    # build legend handles and labels
+    # build unique legend handles and labels
     handles, labels = ax.get_legend_handles_labels()
-    if legend_data != [''] * len(legend_data):
-        labels = sorted(list(set(tuple(legend_data))))
-        idxs = [legend_data.index(d) for d in labels]
-        handles = [handles[i] for i in idxs]
-    if legend is None and len(labels) < 13 or legend is True:
-        ax.legend(handles, labels)
+    handles, labels = np.array(handles), np.array(labels)
+    _, idx = np.unique(labels, return_index=True)
+    handles, labels = handles[idx], labels[idx]
+    if legend is not False:
+        _add_legend(ax, handles, labels, legend)
 
     # add default labels if possible
     ax.set_xlabel(x.title())
@@ -571,3 +574,11 @@ def line_plot(df, x='year', y='value', ax=None, legend=None, title=True,
         ax.set_title(' '.join(_title))
 
     return ax, handles, labels
+
+
+def _add_legend(ax, handles, labels, legend):
+    if legend is None and len(labels) >= MAX_LEGEND_LABELS:
+        logger().info('>={} labels, not applying legend'.format(
+            MAX_LEGEND_LABELS))
+    legend = {} if legend in [True, None] else legend
+    ax.legend(handles, labels, **legend)

tests/test_plotting.py

@@ -43,6 +43,15 @@ def test_line_plot(plot_df):
     return fig
 
 
+@pytest.mark.skipif(IS_WINDOWS, reason=WINDOWS_REASON)
+@pytest.mark.mpl_image_compare(**MPL_KWARGS)
+def test_line_plot_dict_legend(plot_df):
+    fig, ax = plt.subplots(figsize=(8, 8))
+    plot_df.line_plot(ax=ax, legend=dict(
+        loc='center left', bbox_to_anchor=(1.0, 0.5)))
+    return fig
+
+
 @pytest.mark.skipif(IS_WINDOWS, reason=WINDOWS_REASON)
 @pytest.mark.mpl_image_compare(**MPL_KWARGS)
 def test_line_no_legend(plot_df):