In trend chart config, "number of builds" suggests values < 2, but the minimum is 2

In the "Configure trend chart" pane, the "Maximum number of builds to consider" field has a description below it: "If set to a value less than 2 then all builds will be considered." However, clicking the down spinner button will not decrement the value below 2; and if I type the value 1 and focus a different field, then the number-of-builds field gets a red color as if the value were incorrect.

I guess it comes from src/main/resources/charts/trend-setup.jelly.