The Flot widget should support horizontal bars. The Flot JavaScript library provides a "series.bars.horizontal" option for this purpose. Here is an example from the Flot API.txt:

series: {
  bars: {
    barWidth: number
    align: "left" or "center"
    horizontal: boolean
  }
}

The net.liftweb.widgets.flot.FlotBarsOptions trait does not provide any options other than barWidth.