Click on Edit on the top and select the chart. Now in the left side properties pane, click on Data tab. Here expand the Dimension (by clicking on that small play > button to expand). You will see a property 'Limitation'. Click on the drop down value to choose 'Relative Value' or 'Exact value' (based on your data). Then select the last button <= .
In the text field enter 100% (see attachment)
You can do this using Dimension Properties Limitation option or you can also do this by dragging the mouse on X axis from 0 to 80% when you need it.
Or you can use expression like below
If(Expression <= 0.8, Expression, Null())
Note: Replace Expression with your expression using in the chart.
Hope this helps you.