Line Plots · Challenge problem

Eight friends made a line plot of how many marbles each has. One X is hidden under a sticker. The X's you can see are: $1$ X above $1$, $2$ X's above $2$, $1$ X above $3$, $2$ X's above $4$, and $1$ X above $6$. The plot only goes from $1$ to $6$. When the friends put all their marbles together, they can share them equally among the $8$ of them with none left over. Above which number is the hidden X?