Drag a duplicate of your measure to rows:
On the marks card (left side of the picture) click the dropdown that currently displays 'Automatic' and change to 'shape'
On the shape card that appears below that box, click that, and change to triangle.
Next, click the axis of the bottom chart (on my chart, where it says 'Actual Volume') and select 'Dual axis'
Back on the marks card, re-select your second Sum(measure) and adjust the size for your shape.