Use the slider to choose the number of points you want. You can drag any of the points around the screen. The centroid of the points (the mean of the x and y coordinates) is shown in grey. Use the red checkbox to show a general line through the centroid and the residuals. You can move the line around using the red point on the line. Try to make the sum of the squares of the residuals as small as possible. Use the blue checkbox to show the regression line and its equation. The centroid of the data points is shown on the regression line.