Sounds like an interesting challenge. We broke this into two issues...
Issue 1 - Dual axis chart showing a single point of data; We have a potential solution!
We believe this may be do to the aggreation of the measure values:
OPTION 1: Disaggregate the data
1. Navigate to the Analysis menu
2. Uncheck Aggregate Measures
This option will show every row in the data set as its own point on the view and then connect the points with a line.
OPTION 2: Add a dimension to disaggregate the data
1. Drag one or more dimensions to Detail on the marks card
If this does not resolve this issue, attaching a sample workbook that reproduces the issue or a screen shot of the dashboard (cleansed of sensitive data) will be a helpful next step.
Issue 2 - X-axis gets rendered with a label of December 1899
Here is a knowledge base article that may resolve this issue...
Same goes for this, if no resolution, an attached workbook example (cleansed of sensitive data) will be helpful.
Hope these help with your challenge... Please let us know!
We ended up sending our Enterprise Dashboard *.twbx to Tableau Support Services and they were able to diagnose the problem: it turned out that we were blending two different data sources in a single view and were attempting to filter both, simultaneously, on similar fields, and Tableau was unsure as to what to render. Once the filtering was removed from the (second) blended data set, the problem stopped. Thank you for looking into this, however.
Matthew P. Seltzer
Super glad to hear you got resolution!