To determine whether there is a proportional relationship between both variables (height of plant and number of days), we need to establish the constant of proportionality between both at every point along the line.
Hence, as the height increases from one day to another, the increase in height should be the same proportion for every additional day.
Hence, there is no proportional relationship between the two variables and that is further explained by the fact that the coordinates (shown by the various dots) on the graph cannot form a straight line.