You may notice Gallup data from different magazines has different data for the same platform - I believe this is because:
a) I suspect Gallup is probably giving them weekly data rather than monthly. You can compare data in the magazine "Your Sinclair" coverdated March 1990 to that of CVG Feb 1990 if you are curious. Which leads into (b)...
b) For a given "month", some magazines release in that month and others release the month before (IE a mag that says "January" on the cover actually came out in December). I have mentioned the exact publication date or at least month for each magazine where that info is available.
c) Some magazines get older data than others. For instance I noticed Sonic the Comic (which is fortnightly) had data that is more than two months older than another publication.
I will not attempt to "sync" data as that way lies madness given I cannot be sure the sync is right due to my suspicion of weekly chart usage unless examples are as egregious as Sonic the Comic, where I will note it.
This older data is also potentially not as reliable - some rely on one store, and even gallup potentially has issues - please read the three page article in "The Games Machine" titled "Hype Hype Hooray" in issue 5, page 21. I won't post scans of my copy here as I'm not sure on the legality of doing so.
Something else to note is that certain publications mix in non-games in their charts.
I have found charts in the following UK publications for January 1990:
The One
Electron User
CVG
Sinclair User
Your Sinclair
Commodore User
Zero
Amiga Computing (was actually just an apology for not having charts this month)
If you find others I have missed for months already posted please let me know - I will not list the others I have checked in case I missed a chart