I downloaded your examplea and tried to understand how you achieved it. Can you explain a bit about how did the trigger on button connected with the chart. ? and also what does "=(not vShowValues)*-255" mean ?
The button has an action assigned to it to change the value of the vShowValues variable. The chart uses the value of the variable for the text color of the values. So if the button changes the value it's immediatly changed in the chart too. There's no document or sheet trigger involved.
The expression for the text color is black($(vShowValues)). The variable vShowValues is used for the alpha parameter of the black() color function. It determines how solid the color is. black(0) is totally transparant, black(255) is solid black.
The formula =(not vShowValues)*-255 flips the value of the variable between 0 and 255. It treats vShowValues as a boolean. If it has the value 0 then not vShowValues returns -1, if it has a value other than 0 it returns 0. By multiplying the result of not vShowValues (0 or -1) with -255 I get either 0 or 255.