Input box shows a value of a variable, and may allow to change it. It has nothing to do with the fields at all. That means, if you want input boxes to affect charts, you have to use the input box variable values as conditions in chart expressions.
For example, you have field Date, and input boxes for MaxDate and MinDate. You can have expressions like this:
only(if(Date>=MinDate and Date<=MaxDate, Date))
sum(if(Date>=MinDate and Date<=MaxDate, Amount))
count(distinct if(Date>=MinDate and Date<=MaxDate, ID))
Dude! So you just make some whatever variablename in manage variables or while creating input box, then put that variablename in the formula of an expression on whatever chart for whatever field to look at its value.
I just copypasted that MinDate MaxDate expression into a straight table with company, userid, testdate, testresult, and that expression (superficial change of Date to TestDate to match the fieldname on my side). Then I made the two variables, made the two input boxes for the two variables, punched in the dates and watch the chart hop to.
So now I’ve learned the input boxes merely give the hint of what is happening in the expression to plug something in rather than dictate how the expression works. I can write expressions frontwards and backwards, just didn’t understand the mechanics of the input boxes’ link to the other objects. ☺