yes, you can do it if you load an extra field "Measure" with the possible values like "Weekday Stats" and "Weekend Stats", and let users select the value from a Filter.
In the chart, have a conditional expression that shows one formula or another, based on the selected value.
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Check the attached
(html, codepage is 1252, embedded labels, table is @1);
LOAD * Inline "
Field, Visitor, Sales
WeekDay, Sum([Weekday visitors]), Sum([Weekday sales])
WeekEnd, Sum([Weekend visitors]), Sum([Weekend Sales])
Community_214279.qvw 157.2 K
Well, just saw that it is Qlik Sense. Not sure if this will work in Sense or not
UPDATE: Seemed to have worked (with the only problem that we don't have the option to make the filter box have 'Always One Selected Value'). This might become an issue when user selects both weekday and weekend in the field filter.
But check it out
Community_214279.qvf 176.0 K