Change function to plot stacked bar chart to also plot normal bar charts

The function to plot stacked bar charts can be generalized to also plot normal bar charts. Therefor, a new parameter "stacked" will be introduced and the function will be renamed. Depending on the input data-frame and whether it is normal or transposed the ticks at the x-axis and legend entries will be swapped. Columns of the data-frame determine how the entries in the legend are labelled and index is used to label the x-axis ticks.