Bar index of first visible bar in chart

[wrodgers07]

Does anyone know of a hack to get the index number of the first visible bar on the chart (ie, the leftmost bar in the current chart window)? I'm not referring to getOldestBarIndex (which is the first bar in the loaded series) nor getFirstBarIndexOfDay (first bar with the specified date).

I'd like to achieve the equivalent of drawTextPixel for drawLine, meaning I want to draw fixed lines a specific number of pixels from the left edge of the chart window.