A common question we’re getting at HuffPost Pollster lately is: “Why isn’t Libertarian Party presidential nominee Gary Johnson included in your charts?” Occasionally it's more of a screed than a question, but the point is basically the same.

Rest assured, we are not part of some conspiracy to keep Johnson from getting support. Our decisions are based completely on how pollsters are dealing with Johnson -- and that varies a lot.

Fifteen pollsters conducted 25 polls in the month of May. The table shows a summary of how pollsters handled -- or didn’t handle -- third-party candidates in their most recent surveys.

Only three pollsters of the 15 included Johnson in their polls by name: the Democratic firm Public Policy Polling, Fox News and Quinnipiac University. HuffPost Pollster needs at least five polls on a candidate to be able to calculate a trend line before we can include them in our charts, and these pollsters have only one poll each in the last month.

Public Policy Polling

But really, the PPP tossup is the only one with Johnson included that we could use in our chart.

The Fox News and Quinnipiac questions naming Johnson are asked after the tossup question including only Democratic candidate Hillary Clinton and presumptive Republican nominee Donald Trump. Our policy is to use the first tossup question asked, because in survey research we know that how respondents answer earlier questions will affect how they answer subsequent questions. PPP also asked multiple tossup questions, but since it asked the four-way Trump vs. Clinton vs. Johnson vs. likely Green Party candidate Jill Stein question first, it’s the one we use -- with Johnson's and Stein's support counted in our chart's "other" category.

Fox News

The remainder of the polls either only report responses for Clinton, Trump and undecided, or they allowed an “other” or “someone else” response without Johnson’s name attached. Telephone polls, such as the ABC/Washington Post, ARG and CBS/New York Times, typically don’t offer the “other” option to respondents, but will record it if the respondent says they’re voting for someone else.

ABC/Washington Post

Online and automated polls, like Ipsos/Reuters, Rasmussen and YouGov/Economist often have “other” or “someone else” as an option for respondents to choose. Morning Consult, Survey Monkey and Republican pollster McLaughlin don’t offer the “other” option at all.

NBC/Survey Monkey

The answer to why HuffPost Pollster isn’t including Johnson in our charts should be clear at this point: We can’t include him when pollsters aren’t polling on him.

Whether pollsters should be polling on Johnson, or Stein for that matter, doesn’t have an easy answer. Research from 2014 has shown that when pollsters include third-party or independent candidates, the polls overestimate those candidates’ support. But when they don’t include them, the share of the third-party or independent vote is underestimated in their polls.

Maybe this year will be different -- but so far there’s not much evidence to suggest Johnson, who officially became the Libertarian Party nominee last weekend, has a large following. And while the appetite for an option other than Trump or Clinton seems big this year, support for a third-party or independent candidate is actually not much higher than it has been in previous election years.

CORRECTION: An earlier version of this post failed to mention that Quinnipiac University also included Johnson in its poll.