title: Rmarkdown: how to change error messages from R
body: <p>I'm writing in Rmarkdown. My datas are a matrix and a data frame. The issue is that the user selects three options to visualize the plot. For the first selection both datas are degraded as equal rows. For the second and the third selection the user selects one of the columns in both datas for the plot. For some selections it works fine but when there is no value in one of the selected columns it gives an error. How can I print a message like "There is no value for the selections" instead of an error?</p>