Bar chart using dataframe Matplotlib

If your dataframe's index has a name, pandas will distinguish it visually by printing it on a line below the column names. In this case Cluster is the name of your index.

The simplest solution is to leave x alone: it will default to the index if you don't supply a column name:

data_agg_clust.plot.bar(y="KPIPred", rot=70, title="Predicted Volume of impressions")

Another solution is to call data_agg_clust.reset_index to convert the index into a column that can be used for plotting:

data_agg_clust.reset_index().plot.bar(x="Cluster", y="KPIPred", rot=70, title="Predicted Volume of impressions")