It looks like the problem your experiencing may be that the view gets distorted when the product only has sales for a couple of months. When the product contains values for all months, or at least the beginning and ending months, then the workbook you provided shows one line for the Category and one line for the selected product for the entire date range.
It sounds like the desired output is to always show the full category and sub-category lines while also showing a product line for the entire date range selected. The only method I'm aware of that would accomplish this would be to pad your data so that every product has a data value for every month. This would keep the full category and sub-category lines in the view while also showing only the relevant product values.
An alternative method is to build a dashboard containing two charts. One chart would show the Category and Sub-Category view while the second would show only the data points and line for the selected product. I'm guessing this isn't exactly what you need, but it is a method that allows you to see all of the data at once without having to pad the source data. Attached are a screenshot of this method and the workbook with this method applied.
LOD workbook 1.twbx 472.2 KB