With great delight, I was reviewing some of Nick Cox's presentations and notes on data visualization with Stata and have been experimenting with sparkline this morning.

The structure of my dataset is as follows: panel data on >100 farm fields, each of which has 8 strips (4 treatment and 4 control strips). In hopes of comparing treatment and control trends for each farm, I used the following code structure:

sparkline y year if (var1==1 | var2==1) & (var3=="AMS" | var3=="Control") & var4=="z", over(var3) by(field) extremes


The sparklines produced also have a vertical bar in each time period where the outcome variable is observed that seems to represent the range of y over the 4 Treatment and 4 Control strips in each time period (based upon adding the extremes option). However, my question is, how are the starting and ending point for the sparkline determined with the scaling defaults?

I've included a scrubbed example of the corresponding visual.
Array


Thanks.
Maria