Trend lines can be added to most chart type including stacked charts (Bar and Area). The value of each stack is included when the trend equation is calculated. When the separate stackables of a full bar belong to different facts, then a single trend for all facts is drawn. The sum of separate facts (either positive or negative) is included when calculating the trend line equation.
Trend lines are most effect in Scatter, Bubble and Time Aware charts.
A trend line with a single category on the X axis is preferable. In cases where there are multiple categories on the X axis, it might be difficult to analyze the trend if the categories are unrelated.
Pie charts cannot have trend lines.