Your grouping variable can be any of the following:
categorical vector | logical vector | numeric vector | character array | string array | cell array of character vectors | cell array
It can therefore be a vector the same length as total_activity_20_celcius and total_activity_40_celcius. If you want to have two grouping variables, they must be enclosed in square brackets.
gscatter(total_activity_20_celcius, total_activity_40_celcius, [combination_salt, weight_percent_salts])
