Add broken line segments for null values
If the 'y' value of a point is null for a line type of graph, then option should be available to either automatically switch to a dashed line or not to display the line entirely till 'y' gets set to non-null value
Here’s a proposal for how you can extend Highcharts to draw a graphic for null points: http://jsfiddle.net/highcharts/FKSZ6/
In that case we need to look at the next valid value instead of the last. Here's a modified version of the plugin where we look both up and down and interpolate if both values are present: http://jsfiddle.net/highcharts/FKSZ6/
Ashish Shkla commented
What if first value in the series is null?
Your solution doesn't seem to work for the case where first value is null.
Really nice solution .. please find the way to show tooltip on 'x'
Paul Evans commented
A refinement of this idea as follows. Instead of a broken line, use a definite symbol as in this image: http://stats.raveis.com/General/images/missingData.gif. The missing data symbol can be used in different chart types and, since it is unique, it can be unambiguously identified in user documentation as such (An outlined letter "x" represents missing or undefined data)