I managed to produce the same chart by removing the NULL values.
However, I'm having a hard time understanding why Combined CAGO season (calculated field) is shown w/ zero records prior to year 2001. Looking at the table data, there are CAGO seasons that include a large number of reported bands. So, I guess I titled my post wrong. Perhaps, it should have been titled something along the lines of calculated field not correctly showing up chart?
LOL, I'm still fumbling around in the dark w/ Tableau even after watching hours of content from YouTube.
Its due to your axis (Year) starts from 1975 - 2017 so while converting nulls as zero for Combined CAGO season its starting from beginning of the axis.
If you wants to avoid that then don't convert the nulls as zero for that field (Combined CAGO season) then you'll have this view
Let me know if this help.