function shiftedChannels(channels, channel, shift) {
    const channelIdx = channels.indexOf(channel);
    const res = channels.map((c, idx)=>{
        if (idx === channelIdx) return `${c} - ${shift}`;
        else return c;
    });
    return res.join();
}
Return an expression for coordinates into a vector where a given channel
will be offset by [shift].

@param channels the channels to consider
@param channel the channel we want shifted
@param shift  the amount to subtract from the channel.

@returns a string of the form 'x, y-[shift], z' where any one channel can
have the shift applied.