Runtime Error: the operation * cannot accept the arguments "empty string", 1

[Taifun]

That multiplication by 1 seems to be necessary as @TIMAI2 explained here

Creating Multiple Line Graph with Textbox Data

You need to ensure that your textbox values are numbers

My suggestion was this

@Taifun

Make sure the textboxes do contain a number before trying to multiplicate

You also could initialize the textboxes with 0 or add an if statement for both textboxes like this

If is empty Textbox.Text
Then set Textbox.Text to 0

Taifun
PS: I'm not familiar with the ChartMakerPlus extension