Graphical analysis

Hello, I have calculated the interannual average of chlorophyll (2019-2020-2021) and now I would like to generate a curve showing the variability of chlorophyll over this period. What to do ?

It depends a bit on how you have generated your dataset. Probably, it is best to use the Pixel Extraction tool (Raster / Export / Extract Pixel Values) and use the extracted data in e.g., Excel. This allows you to filter the data and show it using graphs. Maybe the Statistics operator is of interest for you too.
Alternatively, you could use the data in QGIS and use its features.