Empty categories from attachment displays are appended to primary charts

While the series data is being combined correctly the categories from the chart attachment displays are being appended to the primary display. Since the categories are identical this results in a situation where the x axis of the chart is repeated a few times with blank data.

E.g. I have a stacked bar chart with year categories. It has three attachment displays with the same years but different data which needs to be stacked on the primary chart. It ends up looking like this:

d
c          d
b          c           d
b          b           c
a          a           b
a          a           a
2018      2019      2020      2018      2019     2020    2018      2019       2020

when it should look like this:

d
c          d
b          c           d
b          b           c
a          a           b
a          a           a
2018      2019      2020

The reason seems to be the removal of this line in the template_preprocess_views_view_charts() function of the charts.module:

$categories = array_unique($categories);

For now I'm attaching a patch that just re-adds that line. However, It has a dedicated commit:

commit dc5bdf351ded5b3b9e552c11dc9c633b8ec7207d
Author: Daniel Cothran
Date: Sat May 5 11:09:13 2018 -0400

remove array_unique on the categories variable

so I'm guessing this was done for a reason and I'm not sure my patch is the best solution.