Line Plots · Medium problem

A line plot shows how much water is in each of $6$ glasses, in cups. Above $\dfrac{1}{4}$ there are $2$ X's, above $\dfrac{1}{2}$ there is $1$ X, and above $\dfrac{3}{4}$ there are $3$ X's. Ben wants every glass to hold $\dfrac{3}{4}$ cup. How many cups of water must he add in all?