Does the Data Library allow me to replace missing data with a constant value?

It is a possible to replace NaN values with any constant value that you wish.

As an example, let's assume you would like to replace all NaN's with -9999.

Before you download your data, select the "Expert Mode" link in the table at the top of the page. Underneath the text already in the text box, enter the following line and then click "OK".

-9999 replaceNaN
All the NaNs in the dataset will be replaced by -9999 and you can now download the data.

-- Michael Bell