Make graphing and evaluating physical before it's done on paper.
Human graph
Build a giant graph on the floor with tape — an x-axis (foil length) and y-axis (brightness). Students physically stand at the coordinate for each data point, and the class "draws the line" by seeing where the line of students falls. Standing on the trend lets them feel the downward slope before plotting it.
Plot with movement
When students transfer to paper, have them place a counter or sticker at each point first (a movable, correctable action) before drawing the line through them. Physically positioning points reduces plotting errors.