Multiple selection variable

[beyzabetulcelik]

My goal:be able to select multiple months for my calculations.

I have a variable, and according to the choosen variable I have a calculation. For example I have a variable that can take values: 'January', 'February', 'March'.

And in my calculation I write:

If($(Var_Month) = 'January', SUM(liter1),
If($(Var_Month) = 'February', SUM(liter2),
If($(Var_Month) = 'March', SUM(liter3),
0)))

 I would like to be able to choose multiple as variable so I can calculate January and February at the same time when choosen. I looked for a multiple selection variable extension but I could not find. Can someone help about it?

NOTE: I don't have data that shows month or I date. I have liter1, liter2, liter3... as a column in the same row.

[madelonjansen]

Since you have the buttons all on the same variable, it will always toggle the value of it, but you need it to append values. I think in this case you would need a variable for every month, and calculate based on that.

Something like:
Rangesum( if( $(vJanuary)=1, sum(Liter1),0), if( $(vFebruary)=1, sum(Liter2),0), ... )

[beyzabetulcelik]

Thanks for the answer.

If I had variables for each month and have 1 option for each of them it looks like this. I cannot unselect a variable when selected. 

My other option is creating something like this. It solves my problem technically but it doesn't look good and it's not user friendly. 

Thats why I am trying to find an answer that user can choose only months (not something like on off). I think I cannot solve this with variable extension thats why I am looking for another extension.