Windowing in the Frequency Domain
The output seems to be okay. I don't see any problem with it. May be you should remove the db conversion part and see the output.
plot(handles. convolution_ axes,f1, 20*log10(abs( convo))); %log plot of the filter
Also, the confusion could be because of the x-axis being viewed from 0 to Fs (0 to N) and not from -Fs/2 to Fs/2 (-N/2 to N/2).
Hope this helps.