There's a jump in user deaths for the map called "ep1_c17_02." The average completion time for the game is 4 hours, 56 minutes, but the average play time is only 2 hours, 48 minutes. Does that mean that part of the game is too hard? The average user is playing for about 3/5 the time it takes to beat the game. The spike in fatality rate seems to occur about 3/4 of the way through the game. That could mean people are abandoning Ep.1 when it gets too tough.
On the other hand, the "Highest Map Played" bar graph shows a linear trend downward. There's not a dip in level completion corresponding to the spike in deaths. Map "ep1_c17_02" loses about 4% of users from the prior map, and a loss of just over 3% to the next map. Most likely, people take a break during a difficult part and then come back to the game later. Additionally, the data show that it's taken at least one person ten hours to beat Ep.1, so they could just be wrecking the curve.
By the way, the stats go deeper than what Valve has chosen to share. Here's Gabe Newell, as quoted in a Eurogamer interview:
Essentially playtests create a proxy for what will happen when the game is being played, but with Episode One we're saying let's stop using the proxy and watch how people play. Rather than having hundreds of playtesters, there are eight million Steam accounts right now, so we'll have eight million playtesters. It tells us which weapons they're using, so we can say "they're not using this weapon, why not?", here's where people are getting stuck "huh, ok, they're not supposed to be stuck here". Here's the stuff they like, here's the stuff they don't like.I picture some guy in a labcoat looking at a clipboard and saying, "This one guy in Boston really seems to love crouching behind things and letting Alyx do all the shooting."