Builtin charts can be extended easily.
Example: adding Coolant Temperature to Speed / Altitude chart.
Go to the Speed / Altitude chart
Click anywhere into the chart to cause the edit arrow to appear:
Click on the arrow and then select "Advanced edit..."
In the chart editor window select "Start"
Then click on the "Get data range" grid
Select "Add another range"
Another box will appear
Click on the "Data" sheet
Click on the column header you want to add: the whole column will change color and the data box will fill up with the column letter.
Click Ok and then Update on the chart editor window.
Chart will be updated with the selected column