ISimpleLineSymbol ldrLine = new SimpleLineSymbol();

IColor color = GetColor(255, 0, 0);          

color.NullColor = true;          

ldrLine.Color = color; // 透明色

ILineCallout lineCallout = new LineCallout();         

lineCallout.Style = esriLineCalloutStyle.esriLCSBase;          

lineCallout.Border = null;          

lineCallout.AccentBar = null;           

lineCallout.LeaderLine = ldrLine;

IMarkerBackgroundSupport pBgSupport = pBarChartSymbol as IMarkerBackgroundSupport;          

pBgSupport.Background=lineCallout as IMarkerBackground;