Weka - Draw a learning curve

I am trying to draw a learning curve for my models, the curve illustrates the relationship between the error and the iteration number or something like this:

I read this post: http://weka.wikispaces.com/Learning+curves

But I have a few problems:

• Unable to retrieve data from database source
• Unable to get the relationship between error and iteration number
• After analyzing the data set and selecting the output format on "GNUplot", I can not save the image when I click the save button, and the following is displayed on the test output:

(generate png:

set terminal png size 800,600

set output "plot.png"

replot)