Different transformer functions on one RRD bug

I wanted to plot a network diagram with Tx-data heading up (above 0) and Rx-data heading down (below 0) for more clarity. Unfortunately I've got both Tx and Rx in the same RRD file, so I got something like:

['data/network/if_octets.rrd', 'tx', 'Transmit', 'bit/s', function (v) { return v*8; }],
['data/network/if_octets.rrd', 'rx', 'Receive', 'bit/s', function (v) { return -v*8; }]

This results in both tx/rx heading up, but this:

['data/network/if_octets.rrd', 'tx', 'Transmit', 'bit/s', function (v) { return -v*8; }],
['data/network/if_octets.rrd', 'rx', 'Receive', 'bit/s', function (v) { return v*8; }]

...results in both rx/tx heading down.

It seems Jarmon applies transformer function only for the first time (considering one RRD file) and ignores it after.

Is it easily repairable?

Thanks in advance

Peter