In slider object properties go to General Tab, here you will find the static step. check that option and set value as 25.
This will give you an increment of 25%, Next set vData variable = 25 set min variables = 0 and max variables = 100 in script
In slider object properties - general tab - assign min and max variables to min and max textboxes
assign vData variable in dimension limits tab - show only values that accumulate to text box of chart object.