When you build your own custom chart, you can use the second axis to display another data item with another chart layout. (bar chart, linear, etc.)
From the report Builder section, select the chart type at the top.
This chart type will apply to Axis 1 by default.
Add your Datasets (actual and / or forecasts data).
From the Axis 1 tab, select one or more data items you want to display.
From the Axis 2 tab, select the other item you want to be displayed with the second chart.
In the Default Axis 2 settings dropdown, select the second chart type:
Any change in the settings is automatically applied in the graph widget:
Example:
Once you've added your data in both axes, you can go back and try with different charts to see which one suits best.