Your Champion Popularity Data is Biased towards balance.
Lets take a look at a few things that I have issues with and how your data biased based off of balance.
If you take a look at the data it clearly has an (Un) Popularity of Monster champs... https://nexus.leagueoflegends.com/wp-content/uploads/2017/09/LOL_CMS_143_Article_04_pssoxt0n6pdln2ya2jwo.jpg But what it leaves out are some of the (popular) monsters: Maoki, Malphite, Kha,Zix, Fizz, and potentially Renekton if we consider him as a monster. So why were these guys conveniently left out of your data that "proves" monster champions are unpopular?
Looking further in depth for monster champions, are they actually unpopular, or does the balance team keep them UP, which keeps them from being popular? Lets take a look at a few champions.
First off Lets look at a few champions who's kits may have slightly changed, or experienced some nerfs/buffs. https://imgur.com/MmO2W6z Janna: Wasn't very popular, oh look enchanters are now FOTM She moves up from being unpopular to played a ton. Rammus. Was decently popular (If you look on the website thats 6.05... that line where he heads to being unpopular is 6.07 aka he go nerfed. Syndra, no real changes to her kit, but I'm pretty sure she saw some gameplay because some oppressive midlaners got nerfed.
Now for the real kick in the balls to show that you control which champ is popular at what time by how you balance the game. https://imgur.com/zfmUK3Y
With no real kit change, jayce went' from "GG trash tier" to "I'm a god" in just a few patches... So lets take a look at a few of the outliers that are "Always popular"
I'm sorry, but I have to go here, Lee Sin, and Yasuo. Why are they popular you might ask? Is it because of their looks http://i0.kym-cdn.com/photos/images/original/000/285/968/016.jpg No.
Is it because of their Balance? https://imgur.com/jKtoemR Yes, yes it is.
What if I told you having the same damage as every other champion, having lots of mobility, and having security in taking a hit or two felt really good? Why you might ask are these guys more powerful than other champs? Well its because their mobility isn't included (as much) in their power budget as other champions. Lets compare them to this poor fucker
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Morde is easily kiteable, low damage when built tanky, can't get to his target easily. Yasuo and Lee are Hard to kite, both had tank builds that were quite successful at one time, and could easily get to, and damage their target. So maybe I should be asking myself, why would I play morde when there are yasuo and lee? That fact alone is exactly what happened with Jayce in the previous image. Why would you play Jayce when x y z do his job better? However, you could give Morde enough buffs that he is 100% pick or ban tomorrow and I guarantee that he will be #1 in popularity.
It may or may not be intentional balance bias, but that is what is causing popularity of champions to be where they are. You keep more popular champions popular by giving them more attention. Keeping them more relavant, which keeps them popular. Lets just admit it that there are some champions like AAtrox, Morde and crew that you just don't even care to balance or have be popular, and like old pre-rework poppy, "shes fine being gutted and if she ever sees pro play she will likely get nerfed" (not quite an exact phrase).
How about some meaningful data like buffs/winrate and popularity changes. That graph would be much more informative than the "Lee Sin is popular, and has been since release". (because we like to keep him relevant)
gets a skin every month? Because she is popular. AKAIK she hasnt been considered "pick or ban" for quite some time.
Also gets skins quite frequently but its been a long time since ive heard any mass uprising about must ban YI.