Calculator with visible decimal point on screen

If you still need this, it looks like the easiest solution (a general outline) is to add a text variable (currentNumText) that can represent the text version of the displayed calculator value (currentNum), use this text variable for the display instead, and create a JavaScript trigger to update the text variable with the new value anytime the currentNum changes.

In the script, you would need to check if the isDecimal variable is true. If so, check if the currentNum variable is currently a whole number or a decimal. If currentNum is a whole number, convert currentNum to a string and append a decimal to the end. If it is a decimal, or isDecimal is false, just convert it to a string (limited to whatever length is appropriate for the display). Save the final string to currentNumText in Storyline. 

The display should now show your modified vaue, with the decimal if appropriate.