Add support for continuous n.group arguments

[tpq]

Providing a continuous outcome to n.group should show a continuous y vs. x plot.

[tpq]

Reviewer writes,

Correlation: While it is outside the scope of this work, for the sake of utility, it would be helpful, when there is a continuous response variable, to plot points within the scatter plot within a y sub-axis for each boxplot in such a way that correlation between each of the balances and the response variable is visible and also be able to highlight/select those balances.