Setting an Automatic Minimum Y to something other than zero

I am building a regression tool where the input data could range from hundreds to millions. The tool produces several charts (like the one attached). As you can see, the lowest y-axis value is well above zero...creating a lot of white space and a fairly flat curve when I use the automatic settings.

While I do know how to change the minimum manually, I'd really like for this chart to automatically set the minimum value of the y-axis (i.e. the x-intercept) slightly below the lowest y value so the curve flattens out towards the bottom of the chart. Essentially, I'm trying to set the x-intercept to the following formula:

X INTERCEPT = SMALLEST Y - (LARGEST Y - SMALLEST Y)*0.1

I've made this regression tool so that is can churn out reports quickly based only input data and I'm trying to eliminate the need for any manual formatting etc.

Thanks in advance,
Chris