Question: Since the internet connection in our house does break down from time to time I set up a little experiment:

For the last two month, one of my machines is pinging google.com on an half-hourly basis. One measurement consists of 50 pings.

I now calculated the mean percentage of packets lost for each hour of the day:
![percent of lost packets](https://i.stack.imgur.com/uzAMk.png)

My questions:

1. Could this peak in the evening be caused by choosing google.com as the ping destination?
2. Would you recommend using an other destination and which?
3. Does this indicate that something is wrong with my connection?
4. What would be a better strategy to measure where exactly the problem in our internet connection is? Our ISP tells us it is working fine so I try to aggregate some proof...

Regards!

Edit: I forgot to mention that the machine is directly connected to the router (no WiFi). And the router is pinged as well, with no packet loss at all.

Answer: 
Unfortunately you really have not provided enough information to work out where the problem is. To answer as best as I possibly can with the limited information provided:

1. If my experiences are anything to go by, pinging Google is normally quite a good bet, as they design their network to be as fast as possible. Also as ICMP is prioritized, evening peak probably does not make a significant difference - particularly in terms of packet loss - which I'd argue should be 0.
2. Google is a good destination, but to get a better idea of what is going on
you may want to additionally try pinging your gateway, and if they allow it,
your providers DNS, mail or web server. This will help show where the packet
loss is creeping in. Realistically though, at the level of packet loss
you are seeing, look at downloading MTR (or WinMTR) and running that at peek
to get a better idea of where packet loss is coming in.
3. Subjectively, 5% packet loss is at the top end of acceptable for a Wifi
based network - assuming you are not saturating your network. On the flip
side, I get upset about 0.5% packet loss on my fibre connections - as a 
point of reference, loosely speaking, less then 1% is OK for VOIP, above  

that not so much. If you expect to be able to use Skype or Viber or 
what have you on your connection then 5% packet loss is not OK. For
just Web browsing it may just about suffice.
4. As an ISP, I want to see the results of an MTR, which show the latencies
and packet loss between the destination - this helps me look where the
bottleneck might be and is a good first step. I would also like to know
when the test was done so I can correlate it with the customers other
usage and what is going on on the system. The packet loss graphs you 
have done are useful as well, but not in isolation. 

As a client, my ISP has been unable to excuse away my graphs, which plot
packet loss (I do it for 250 pings, once a second over 5 minute intervals,
combined with minimimum, average and maximum latencies for those pings). I
also have a set of graphs showing my utilization of the link, and have
sets of graphs showing local (ie very close to me), and to another POP
they own of specific interest a few hundred KM's away.

Other observations:

It looks like your latency increases in the afternoons - which means the first places I would be looking are if the problem is WIFI when everyone around me is using it. After ruling that out I'd start questioning my ISP about oversubscribing the connection.
This is typical of most residential ISP accounts. You're seeing a peak due to network congestion as people get home after work, then go on-line for the whole evening. This sort of evening peak is especially pronounced in high-tech communities with lots of on-line gamers (like where I live, here in Redmond, home of Microsoft.)