The idea that the DNC "rigged" the primaries is false.

I voted Bernie in the primary, too, but it is very important not to take the claim in the WikiLeaks graphic at face value.

This July 2016 New Republic article debunks the claim: No, the DNC Didn’t Rig the Primary in Favor of Hillary:

Wikileaks’s tweets conjured dark and menacing conspiracies, but these are not borne out by the emails themselves. Take the group’s claim that the “DNC knew of Hillary paid troll factory attacking Sanders online.” The highlighted email isn’t some secret communication laying out nefarious plots. It’s a summary of a panel discussion on Fox News Sunday. But forget the emails for a second. The main problem with the notion that the DNC rigged the results for Clinton is that it requires one to assume the improbable. The DNC had no role or authority in primary contests, which are run by state governments. Clinton dominated the primaries. The DNC, through state parties, had a bit more influence over caucuses … where Sanders dominated Clinton. None of the thousands of leaked emails and documents show the DNC significantly influencing the results of the nomination. Furthermore, if it is true that last fall Clinton campaign chair John Podesta tried but failed to have DNC Chair Debbie Wasserman Schultz sacked, the underlying premise of the entire WikiLeaks dump—that Wasserman Schultz machinated to deliver Clinton the nomination—is hard to believe.

See also Kurt Eichenwald's fiery but comprehensive Newsweek article, The Myths Democrats Swallowed That Cost Them the Presidential Election:

Almost every email that set off the “rigged” accusations was from May 2016. (One was in late April; I’ll address that below.) Even in the most ridiculous of dream worlds, Sanders could not have possibly won the nomination after May 3—at that point, he needed 984 more pledged delegates, but there were only 933 available in the remaining contests. And political pros could tell by the delegate math that the race was over on April 19, since a victory would require him to win almost every single delegate after that, something no rational person could believe.

In other words, the emails were supportive of the at-that-time mathematically-certain Democratic nominee, Hillary Clinton. Eichenwald continues:

Once only one candidate can win the nomination, of course the DNC gets to work on that person’s behalf. Of course emails from that time would reflect support for the person who would clearly be the nominee. And given that their jobs are to elect Democrats, of course DNC officials were annoyed that Sanders would not tell his followers he could not possibly be the nominee. Battling for the sake of battling gave his supporters a false belief that they could still win—something that added to their increasingly embittered feelings.

He concludes:

Bottom line: The “scandalous” DNC emails were hacked by people working with the Kremlin, then misrepresented online by Russian propagandists to gullible fools who never checked the dates of the documents. And the media, which in the flurry of breathless stories about the emails would occasionally mention that they were all dated after any rational person knew the nomination was Clinton’s, fed into the misinformation. In the real world, here is what happened: Clinton got 16.9 million votes in the primaries, compared with 13.2 million for Sanders. The rules were never changed to stop him, even though Sanders supporters started calling for them to be changed as his losses piled up.

In a detailed state-by-state analysis, FiveThirtyEight declared (at the time) that The System isn't Rigged Against Sanders:

Realistically, if you throw everything together, the math suggests that Sanders doesn’t have much to complain about. If the Democratic nomination were open to as many Democrats as possible — through closed primaries — Clinton would be dominating Sanders. And if the nomination were open to as many voters as possible — through open primaries — she’d still be winning.

One important point, mentioned also by some of the articles above:

The DNC conspiracy claim was Russian propaganda, picked up by Bernie supporters, and later by Trump supporters

US Officials Say They Have Proof Russia Provided WikiLeaks with Hacked DNC Emails:

Three US officials confirmed this week that the intelligence community had conclusive evidence after the election that Russian cybercriminals provided WkiLeaks with hacked information from the DNC, reuters reports.

The timing of the release of these emails, the night before the Democratic National Convention, was clearly planned for maximum damage to the party.

Bernie's campaign manager even supported the narrative:

Weaver said the emails showed misconduct at the highest level of the staff within the party and that he believed there would be more emails leaked, which would "reinforce" that the party had "its fingers on the scale."

Later in the general election, Trump picked up on the claim and used it against Clinton (one tweet of many to choose from):

President Obama should ask the DNC about how they rigged the election against Bernie.

This narrative was reinforced by Russian Twitter bots and fake Facebook personas:

An investigation by The New York Times, and new research from the cybersecurity firm FireEye, reveals some of the mechanisms by which suspected Russian operators used Twitter and Facebook to spread anti-Clinton messages and promote the hacked material they had leaked. On Wednesday, Facebook officials disclosed that they had shut down several hundred accounts that they believe were created by a Russian company linked to the Kremlin and used to buy $100,000 in ads pushing divisive issues during and after the American election campaign.

The New York Times article continues, detailing the focus these (suspected) Russian sources had on the "corrupt Hillary" narrative:

Several activists who ran Facebook pages for Bernie Sanders, for instance, noticed a suspicious flood of hostile comments about Mrs. Clinton after Mr. Sanders had already ended his campaign and endorsed her. John Mattes, who ran the “San Diego for Bernie Sanders” page, said he saw a shift from familiar local commenters to newcomers, some with Eastern European names — including four different accounts using the name “Oliver Mitov.” “Those who voted for Bernie, will not vote for corrupt Hillary!” one of the Mitovs wrote on Oct. 7. “The Revolution must continue! #NeverHillary”

Finally, it is naïve to think that Bernie would have had an easy time against Trump

From Eichenwald's Newsweek article quoted earlier:

So what would have happened when Sanders hit a real opponent, someone who did not care about alienating the young college voters in his base? I have seen the opposition book assembled by Republicans for Sanders, and it was brutal. The Republicans would have torn him apart.... The Republicans had at least four other damning Sanders videos (I don’t know what they showed), and the opposition research folder was almost 2-feet thick. (The section calling him a communist with connections to Castro alone would have cost him Florida.) In other words, the belief that Sanders would have walked into the White House based on polls taken before anyone really attacked him is a delusion built on a scaffolding of political ignorance.

Plenty of other dirt out there on Bernie, if, like the Republicans, you want to spin it that way:

Consider the claim that BernieWouldaWon. This is based on many things, but there's a base assumption that he wasn't "flawed" or "tarnished" or whatever word we are using today. That Clinton's "flaws" brought her down. Okay. Let's play "Bernie Woulda Been Flawed". How would that go down? Well, here's a few samples. Bernie the Liar. Bernie the Creepy Sex Freak. Saunders the Stalinist. Jane Sanders, or, Bernie's College Plan is Terrible. Sanders the Racist Nativist Sanders and Trump are Just the Same, but Maybe Bernie's Worse.

(Read the article for details and references for each. Remember, that author doesn't believe these things about Bernie, but is giving examples of how they could have been used against him.)

EDIT: The lawsuit against the DNC was dismissed in August 2017

Florida judge dismisses fraud lawsuit against DNC:

“To the extent Plaintiffs wish to air their general grievances with the DNC or its candidate selection process, their redress is through the ballot box, the DNC’s internal workings, or their right of free speech — not through the judiciary,” Judge William Zloch, a Reagan appointee, wrote in his dismissal. “To the extent Plaintiffs have asserted specific causes of action grounded in specific factual allegations, it is this Court’s emphatic duty to measure Plaintiffs’ pleadings against existing legal standards. Having done so . . . the Court finds that the named Plaintiffs have not presented a case that is cognizable in federal court.”

To be clear, the judge ruled that none of the plaintiffs had "standing", not necessarily that their claims of partiality were necessarily false: