chart/bars: add the bar-width{,s} and bar-offset{,s} options

    {"data-y", bind(&data_load, _1, &c->y)},

    {"data-x-offset", bind(&data_load, _1, &c->xoffset)},

    {"data-y-offset", bind(&data_load, _1, &c->yoffset)},

    {"bar-width", bind(&data_load, _1, &c->sizes)},

    {"bar-widths", bind(&data_load, _1, &c->sizes)},

    {"bar-offset", bind(&data_load, _1, &c->offsets)},

    {"bar-offsets", bind(&data_load, _1, &c->offsets)},

    {"limit-x", bind(&expr_to_float64_opt_pair, _1, &c->scale_x.min, &c->scale_x.max)},

    {"limit-x-min", bind(&expr_to_float64_opt, _1, &c->scale_x.min)},

    {"limit-x-max", bind(&expr_to_float64_opt, _1, &c->scale_x.max)},