The chart showing media attribution over time displays for each period the share of KPI (the percent of dependent variable) that can be attributed to different media channels.

The baseline is the share of KPI that is attributed to non-media factors, i.e. seasonality, trend, other independent variables you used in your model. In other terms, the baseline is the estimated share of KPI that would happen in the absence of media spend.

For the different media channels, look for the most important one. Does it align with your domain experience? Look also for consistency from one period to the next; the attribution mix should generally change slowly.