How to merge two data sets in Tableau via relationship

Question

I'd want to integrate two data sets and generate a combined bar/line chart. The columns should be months, and the rows should represent the total values for that months. However, one of the data sets should be displayed as a bar chart, while the other should be displayed as a line chart. This is how it should look:

I have the following data sets:

data_set1 = [{'value':10,'date':2020-01-01},{'value':15,'date':2020-01-02},{'value':5,'date':2020-01-03},{'value':10,'date':2020-01-04},{'value':120,'date':2020-02-01}]

data_set2 = [{'value':100,'date':2020-01-01},{'value':200,'date':2020-02-01}]

I've been able to create the bar chart (see below): 

However when I try to add the values from the second data set by also adding it to the columns section, I get the following error:

"In order to use fields from dataset 2, a relationship needs to be created with dataset 1. Select Data > Edit Relationships to open the Relationships dialog box."

I tried to create a relationship between these two data sets by the following process:

1. Set the Primary Data Source as data_set2, since that is the smaller dataset.
2. Selected Custom to create a custom relationship
3. Selected Add and set both the 'Primary Data Source Field' and the 'Secondary Data Source Field' as Date