nid is getting plotted instead of the decimal field value

I'm seeing some very strange behavior when I try to plot the 'value' of a decimal field using Flot and Views.

I have a simple configuration with two fields: Post Date (date) and Amount (decimal). I have the Post Date field set to the X-axis and a the Amount field to the Y-axis.

When the function of the X-axis value field is set to 'count', a get a nice plot showing me how many nodes meet the filter criteria for each day. But, when I switch the function for the amount field to 'Sum', I get a plot showing the sum of the Node IDs of each of the nodes meeting the filter criteria for each day. Instead of getting the sum of the amount values which should be 625.00 + 340.00, I'm getting the sum of the node IDs which are 3425 + 4352.

I've tried this with and without aggregation with different filter criteria and it seems to be doing the same thing every time.

Am I doing something wrong or is this a bug? Can someone provide some guidance as to how to plot 'values' instead of 'nids'?