I suggest to add some very common buy/sell indicators for stock charting, in particular the treatment for "moving average curve"
Buy / Sell Indicators:
1) Moving average curve by adopting historical close price (say moving average of past 10 / 20/ 50 / 100 /250 trading days, i.e. days with trade) and the curve should be plotted as a smooth curve instead of ladder-like flat cuve;
2) RSI (relatvie strength index to show oversold or overbought)
3) MACD, etc.
Implemented as plugin for Highstock 1.3.9+, see: http://www.highcharts.com/plugin-registry/single/24/Indicators
I had to display a smooth curve, as your 2nd point request, and this is what I did : http://jsfiddle.net/6gXfu/ using the groupPixelWidth.
You can change groupPixelWidth and the zoom to see how it does. It is quiet nice.
I hope this has been of some help !