Following Jon's comment to my previous post, let's look back at how SfE started.
In December 2006, after a 10 months trip across the Andes, I had a few weeks to kill and decided to learn some VBA programming.
This is how I found 2 posts on the excellent site "Daily Dose of Excel" dealing with in-cell charting.
If you look at the LineChart() code, you will see that the variables names are still the same and the use of the Worksheet_Calculate event introduced by Jon Peltier has not changed either.
At the time I already knew about Edward Tufte's work, and I immediately imagined in-cell bulletcharts and barcharts in grey shades and thin red lines to highlight target or reference values.
Pareto and Cascade naturally followed as I hade been using them almost daily in my job.
Scales quickly show to be necessary to bring some context around the charts.
Nixnut logically added Boxplot, AreaChart and the brilliant HorizonChart.
Surprisingly, the simple VariChart was not obvious to me, maybe because I was using the Rep() formula hack.
As I knew very little (and still do !) about VBA, I searched and found what I needed to create the add-in, deal with arrays, etc...spending hours browsing through VBA forums.
As I have been travelling extensively during the last year between Paris and Bucharest, I used the flight time and quiet hotel night for programming and endless testing.
SfE started with those 55 lines of code published by Rob van Gelder and improved by Dick Kusleika, Jon Peltier and other contributors, to reach several hundreds lines for 14 different types of charts today (and a couple more in the pipe).
Therefore, from what I know, only Rob van Gelder should be in position to claim the "invention" of in-cell charting for Excel.
Said this, Bonavista and Bissantz might have started their Sparklines solutions earlier and from different inspiration.