how can i add trendline or best fit line on a scatter plot which has multiple set of data. for example, i had 3 set of data for 3 different depth (0cm,15cm and 30cm) and i plot all three in a scatter plot. is it posible if i want only a line for describe all three depth despite of having a line for each color (3 line for 3 depth). or if i made all 3 depth together in one set of data and get a trendline from that scatter, how i can differentiate which point is 0cm, 15cm and 30cm?