I appreciate your help. Unfortunately it's not quite what I am looking for. The data set you had used was a summary table from 01/01/2016 to 12/31/2018. So it already contains the months needed to create the cumulative line chart. However the data set I am using contains only the month of October 2016, and none of the other months.
Based on this information, is that something that can still be done in Tableau? Or will I need to create a summary table like you did?
Tableau does have the ability (under certain conditions) to "densify" the data (creating NULL marks for data between to values). However, the date column does need to have your final date (31/12/2018) in there. Tableau's domain completion let you densify data for (what are known as) Range Aware Pills. Range Aware Pills are either Dates or Bins, where Tableau knows the MIN and MAX and the increment.
So you wouldn't need every month in the data. The below table would let you do this
Date Value 10/01/2016 1 10/01/2016 1 10/01/2016 1 10/01/2016 1 10/01/2016 1 10/01/2016 1 10/01/2016 1 10/01/2016 1 10/01/2016 1 31/12/2018 0
In the attached, I've used the "Show Missing Values" to trigger the domain completion
And then used Table Calcs to access these values (which are NULL, but accessible). Again if you wanted the Viz to go back before your first date, you'd also need that date in the data too.
I don't know what source your data is from, but you just need a 2 row table (with your first and last display date) which you could then union to your data, and then you can use the above method.
Hope that helps.
Data Densification on Dates.twbx 27.0 KB
Thank you all so much for your help!