Trying to show prior month data against current month

[Kivel]

Hi, 

Let me start by saying I am a “business” user of looker. I have no experience writing with LookML and only have a limited knowledge of SQL. I have done some simple custom table calculations, dimensions, and measures in Looker, but those are the extent of my knowledge. 

I have a graph which shows a month-to-date media spend along with a forecasted spend thru the end of the month. The graph has two lines representing these. 

I need to show the prior month’s spend to this same date (in terms of number of days that month), but without the forecasted spend for that prior month (although, if it is impossible to do this, it would be acceptable to also show that line representing the prior forecast). The spend will be filterable by Vendor or Geo. 

Here’s an image of what I am trying to accomplish:

The attached document has a working (fake) data set. It also provides a list of some custom calcs that were created for a single month diagram - that just has the month’s spend-to-date and the forecast. 

The original diagram was made by someone who is no longer available to assist with this. That diagram was also based on actual data which cannot be shared. web_data_set_4-12