Number of genomes shown is incorrect for scatterplots

[jameshadfield]

Scatterplots will not render nodes which don't define values for the selected x and y variables, however this information is not taken into account for the info-header "Showing X of Y genomes..."

Potential solution: Technically these nodes are bing rendered off-screen, to allow easy transitions when we change to x,y variables for which they are defined. If we could instead hide them my modulating their visibility (in redux state) then this issue would be resolved.