Because the slope doesn't tell you any actual points on the line. It only tells you whether the line tilts up or down, and how steeply. But the line could be way up there or way down under, with the same slope. You can't actually DRAW it until you know some point on it, and the y-intercept gives you one.
Whisker plots is not that hard you just have to no the Quartiles the center of the graph is Q2 the first dot to the left is the lower quartile the last dot to the left is upper quartile so the second dot is Q1 and the 4 dot is Q3
