Trend line as expression based on Field A - Field B issue.

[wiz_nerd]

I am trying to use the expression based on the difference of two fields which I am displaying as Line to see the trend.

Trend is based calculated from

Blue value - Red value = green (displaying as a line to see the trend)

In an overall trend the  output of the difference should not be negative.

A)when I am selecting the part of the combo chart the values are recalculated based on the selection and the trend is displaying negative values. Is there a way to prevent this and stop the trend to recalculate and just show the trend as overall trend.

Overall Trend Values

When selection a part

Negative Values based on recalculation.