if you want to "freeze" Store type as a second dimension and change only Product Type by other two alternate dimensions, it is not possible. User can freely change whatever dimension he wants. In such cases I prefer to use a "note" below chart heading with info that second dimension is rather to be not changed due to unexpected results.
Or if you do not want to use this approach, you can create input object with drop-down list using variables and call predefined variable values.
BR
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ECG line chart is the most important visualization in your life.