Is there any way in which I can create a drop down (with the options of QTD and YTD) on a Webi Report, which I can then use to control whether a chart is visible or not?
If you have users who have the ability to use input controls in view mode, this is pretty quick and easy and can be used to have the option to give users different visualizations, look at row level vs aggregated blocks without having to launch child reports, etc. in addition to your requirement to look at a block with one calculation vs. another:
Create a block with your YTD column and a block with QTD column.
Make a new Variable called “View” or something like that, make it =“YTD”
Create an input control with Dropdown using your View variable and add a custom value to the list in creation for QTD that will be in addition to your YTD value.
Lay the blocks on top of each other and right click your block with YTD to check the hide when formula is true to =[View] = “QTD” and do the same on the QTD block for where [View] = “YTD” so that as the dropdown/radio button etc is used to select one the other is hidden.