A simply X-Y axis line graph is a good choice for this. On the X axis she will have the height of the plant (make sure to label the axis, and identify the units of meansurement -- e.g., centimetres or inches). 

On the Y axis she will have thirty spaces, one for each day of the month -- make sure to label that axis as well, and to clearly define the time period. 

She will end up with a line that starts at 0 and extends upward diagonally, which will give a really clear illustration of the plant's growth.