I am building a web-based analytics platform. I am looking to plot a line chart that represents the number of seconds it took for me to reach a live service agent via phone on a given day.
In the y-axis, I have seconds. In the x-axis, I have date. I place one call per day, so I have one data point per day.
If I call and get an answering machine, I am left with a null data point, because I can't tell how long it took to reach a live agent, since I never reached one.
Currently, I am skipping over that date and just connecting the data point with the next available data point via the line (to show a trend).
However, these null values need to be represented in the chart because I need to indicate that I did indeed make an attempt to reach a live agent every day. It is also valuable to quickly be able to see when I was directed to an answering machine.
There is an added level of complexity here in that I am plotting this for multiple companies simultaneously on a single chart. A friend had suggested that instead of a line chart, I use a bar chart and represent each call with a bar value and that a null data point could be represented with a value of -1. This solution works great if I am only plotting the data for one company, but having multiple companies makes it less attractive since too many bars clutters the chart and makes the trend for a given company a bit tougher to decipher.
My best idea so far has been to represent each null value as a zero and simply change the marker on the datapoint to be a red "X" to indicate that it is a null value and not an actual zero value. The feedback I have gotten is that it's not an accurate representation and that it messes up the trend of the chart by lacing it with spikes. Here is an example of my idea:
I am really struggling with this issue and appreciate any thoughts or feedback.