Create selection function$$

Create a function called on$\_$selection which is tied to the heatmap with an $.$observe call on the heatmap's selected key.

The function should only allow for one selection at a time $($i$.$e$.,$ no multiple cell selection$).$

The function should change the data used in the bars plot based on the cell selected by the user.

NOTE: Be sure to account for the fact that some cell's will result in empty subset dataframes $--$ this will lead to an error. To fix this, make sure the bars plot has the value of an empty string $([\text{'}\text{'}])$ on its x$-$axis $("$B$")$ and the value of $[0]$ on the y$-$axis $("$Mean B$")$ when the subset dataframe is an empty string.