A little light on details, but I think you might want to create a standard hierarchy... there are knowledge base articles about how to do that. Your first calculation/highest level in the hierarchy would be called Year and would contain
Your next level in the hierarchy would be Month (or Month/Year), and would read:
IF YEAR([YourDateFieldHere]) = 2018 THEN MONTH([YourDateFieldHere])
If you're not talking about drilling down in the true sense, and you just want to have a bar/mark for 2017 displayed next to monthly 2018 marks, you can probably just use this calculation on its own.
Alternatively, you might want to show the month name instead of the number, in which case you will need to use
IF YEAR([Order Date]) = 2018 THEN DATENAME('month',[Order Date])
ELSE STR(YEAR([Order Date]))
Finally, make sure you sort this Ascending by [YourDateFieldHere] either Minimum or Maximum. This should ensure your values appear in the correct order.
If that's not clear or doesn't answer your question, perhaps you could give us a bit more context.
I think you know exactly what I want. You can do this in excel because there's a little plus everywhere, but then you can't do it here in the same manner. Since this is my first real day making graphs in Tableau, I can't say for sure, but generally tableau deals with these mixed cases with formulas it seems.