The short answer is no, it is not possible. The RRDTool library does not contain this feature.
The longer answer is that, if you really need this, there is a way to sort of achieve it programmatically. You would need to generate the graph (without labels) and store it into an image file. Then, use something like the GD library to modify the image yourself and add the labels -- you can work out where to add the labels by combining the size data you get on generating the graph (this gives the coordinates of the graph area and the Y-axis) with a second call to RRDTool to get the 'last' values of the variables (to identify where to locate the labels vertically). This may be too much work, though.
It may be that a different graphing tool is more appropriate for your needs? You can always use RRDTool to hold the data, then extract it with a 'fetch' or 'xport' call, and pass this to a different graphing function to be rendered.