TAP values in I/O Graph

Hi, I've got a dissector in LUA for my own protocol and a TAP to calculate other values : for example number of packets send :

function tap.packet(pinfo, buffer)
counter = counter + 1 
end
function tap.draw(t)
window:append("number of packets : " .. counter .. "\n")
end

Is it possible to plot the counter value in the wireshark I/O Graph tool ? I guess I need to put counter in a field but I don’t find how to do that.

Thanks,