Fix unreadable labels in plot_confusion_matrix in case of imbalanced …

…data and show_normed=True (#504)

* Change bg color to follow normed values like label

Use the normed conf mat for background, if normed is set, as label coloring also follows normed conf matrix.
Fixes unreadable labels in imbalanced datasets.

* Add bugfix description

* Fix typo & add link to PR

docs/sources/CHANGELOG.md

@@ -25,6 +25,8 @@ The CHANGELOG for the current development version is available at
 
 ##### Bug Fixes
 
+- Fix unreadable labels in `plot_confusion_matrix` for imbalanced datasets if `show_absolute=True` and `show_normed=True`. ([#504](https://github.com/rasbt/mlxtend/pull/504))
+
 - Raises a more informative error if a `SparseDataFrame` is passed to `apriori` and the dataframe has integer column names that don't start with `0` due to current limitations of the `SparseDataFrame` implementation in pandas. ([#503](https://github.com/rasbt/mlxtend/pull/503))
 
 ### Version 0.15.0 (01-19-2019)

mlxtend/plotting/plot_confusion_matrix.py

@@ -66,10 +66,10 @@ def plot_confusion_matrix(conf_mat,
     if figsize is None:
         figsize = (len(conf_mat)*1.25, len(conf_mat)*1.25)
 
-    if show_absolute:
-        matshow = ax.matshow(conf_mat, cmap=cmap)
-    else:
+    if show_normed:
         matshow = ax.matshow(normed_conf_mat, cmap=cmap)
+    else:
+        matshow = ax.matshow(conf_mat, cmap=cmap)
 
     if colorbar:
         fig.colorbar(matshow)