A trend line fits a straight line through the marks, so you can see the overall direction and how well the marks follow it. Drag Trend Line from the Analytics pane onto the view, or choose Analysis > Trend Lines > Show Trend Lines.
Trend lines need two continuous axes: two measures (a scatter plot), or a measure against a continuous date. A discrete axis of headers has no numbers to fit a line to, so Tableau won't draw one.
With a dimension on Color, Tableau fits one line per color, which compares the trends of each group.
Hover a trend line to see its equation and two numbers:
- R-Squared, from 0 to 1: how much of the up and down of the marks the line explains. Near 1 means the marks hug the line.
- P-value: how likely a slope this steep would appear by chance. Below 0.05 is the usual bar for "real".