Solution: Line Plots
Answer: KOOL AID

Written by Alan Huang

This is a puzzle about video game series with weird numbering conventions.

Each graph represents a series, with games always shown at the x-coordinate for their original release date. Numbered installments are shown with a dot at the corresponding y-coordinate; when the numbers are high, the scale is logarithmic. Those with both numbers and additional words show rebus icons attached to the dot. Titles without a number use icons only; they are given a y-coordinate by interpolating between neighboring numbers, which may be confusing at first, but should eventually become clear. Not every game in every series is depicted — I tended to drop ones that would have required their own icons not shared with any other games — but most of the mainline entries in each series should be covered.

A roller coaster icon is provided as a starting hint. The last two are RollerCoaster Tycoon 4 Mobile and RollerCoaster Tycoon World.
Half-Life 2: Episode One and Two are represented as 2.1 and 2.2. Half-Life 3 has been rumored for over a decade (Gabe Newell pictured) but never confirmed.
Mario Party has straightforward numbering up to 10, but this graph introduces Game Boy Advance and Nintendo DS icons. After 10 come Mario Party: The Top 100 and then Super Mario Party.
This graph includes every game titled James Bond 007. It unfortunately breaks the rule by not including icons for all the subtitles, but they don't really line up with the icons used elsewhere and would have been hard to clue. Hopefully the constant line at 7 is suspicious enough to give away the series.
One of the more obscure and less interesting graphs here. Europa Universalis: Rome was released in 2008.
Mostly straightforward, except that Code: Veronica and Zero came out before 4.
This one might be tricky to identify despite how well-known it is. I considered including all the World of Warcraft expansions up to the present, but as above they would have been hard to clue.
EA rebooted the series in 2015. Our puzzle from last year references the most recent game.
Now we get into the more interesting icons. CivCity: Rome was released in 2006. There were also two Civilization Revolution games for consoles, as well as Civilization: Beyond Earth.
In order: 1, 2, 358/2 days, Birth by Sleep, 3D: Dream Drop Distance, HD Remixes 1.5, 2.5, 2.8 (Final Chapter Prologue), and 1.5 + 2.5 (= 4), and 3. The icon above for revolution is used here as day. I opted not to include the subtitle on Dream Drop Distance as it acronyms to 3D anyway.
Mass Effect Galaxy came out in 2009 for iOS; Mass Effect: Andromeda came out in 2017.
After IV: Black Flag, we have Unity (1), Origins (0, 0), then Odyssey.
BioShock Infinite was (asymptotically) released in 2013.
Despite the jumping around, each individual installment here should be pretty straightforward. The high-numbered ones are 1942, 2142, and 1943.
Dragon Age: Origins, II, and Inquisition.
The main series counts up to 4: Modern Warfare, which splits off its own series of three games. Then it continues with World at War, Advanced Warfare, Infinite Warfare, and WWII. The separate line below has a different color because it represents the four numbered Black Ops games.
Another boring graph, but the early 80s start and very early MMO installment (Ultima Online, 1997) can serve as clues. Ultima VII had a Part Two.
In order: Super Mario Kart, 64, Super Circuit, Double Dash, DS, Wii, 7, and 8. (8 Deluxe not pictured.)
The first icon here means Arena. V: Skyrim was released in 2011, followed by The Elder Scrolls Online.
Final Fantasy IV and VI were released as II and III in North America, after which VII was not altered. X had a direct sequel, as did XIII, but Type-0 is (arguably) in the same series (using the icon that meant code above). XIV was relaunched as A Realm Reborn, and World of Final Fantasy was released in 2016 shortly before XV. (All the Bravest not pictured.)
Age of Empires II: The Age of Kings released in 1999 and was given an HD Edition fourteen years later.
The North American version of Super Mario Bros. 2 was famously a completely different game rebranded as Mario. After Super Mario Bros. 3 we have Super Mario Land and World. 64 received a remake as a launch title for the DS. After Sunshine we have Galaxy and Galaxy 2, and most recently, Odyssey.
The little-known 1998 Pokémon Stadium was released only in Japan. Its sequel was released as Pokémon Stadium in North America, followed in the second generation by Stadium 2 (instead subtitled Gold / Silver in Japan). In generation 3, abandoning the Stadium name, we have Colosseum, XD, and finally Battle Revolution.
Many yearly sports series switched between 200X and 9X / 0X / 1X, but Madden takes the cake by also having 1, 2, 64, and 25 installments.
Ragnarok Online 2 was canceled and rebooted in 2012. Several spinoff games released in the interim included Ragnarok DS and Ragnarok Odyssey.
The classic Mega Man series spanned eight games up to the mid-90s. A separate series using Roman numerals was released for the Game Boy. The darker blue line represents the X series (interpreted as 10), while the red is the Zero series. Finally we have the "modern" continuations 9, 10, and 11.

Since graphs are paired up, write the names of the series next to each other as clued by the left-aligned enumerations, and read off the unique letter that is in the same position in both titles. A web search for LAVA IS BOILING will turn up a TV Tropes page, so the answer is KOOL AID.

Author’s Notes

There are unfortunately a few "filler" series that were included to get the right letters, but I was able to pick most of them based on having interesting graphs or icons shared with other series. As a result, though, the puzzle ended up pretty long. Hopefully identification is easy enough to compensate for that; even testsolvers who didn't play many video games reported that they were able to find most of the ones they didn't recognize by looking for games released in certain years with the given enumerations.

The title of this puzzle is a reference to an MIT Mystery Hunt puzzle named Plotlines (inspiration, sequel, sequel), although this puzzle shares little in common with it. I wanted to title it Hotlines as a subtle Easter egg (a term that here means "cute footnote to put in the solution") referencing the game Hotline Miami 2: Wrong Number, but was overruled after some discussion.

It is interesting to note that the Super Mario series in particular also provides an ordering of the Earth, Sun, Milky Way, possibly a deity (Maker), and the Mediterranean Sea, presumably in order of increasing size, as well as in relation to various numbers. Up to three brothers are smaller than the world, but the world is no larger than 64. Any conclusions to be drawn are left as an exercise to the solver.

Shortly before hunt, it was announced that USB 3.0 would be rebranded as USB 3.2 Gen 1, USB 3.2 Gen 2, and USB 3.2 Gen 2×2. Maybe that should have made it into this puzzle.