@Stan Taylor nailed it...
The velocities on your chart are single samples and are important but cannot get us all the way to the goal.
Yes, your groups are just as important as your velocities when testing at short range and extrapolating to longer ranges.
Tuning a gun is done because it makes a very big difference. External ballistics and group performance at range are non-linear and that regression straight line on the plot is just one example of this. When you tune your rig, the recipe includes lots of parameters but let's make this a shorter talk and stick to the simplified version that says you are searching for the correct charge and depth.
The speed and charge relationship is basically linear but those waves are what we are searching for in terms of tuning. The impacts and groups are similar in that they don't respond to incremental changes in powder in a linear way. We are trying to find the advantages of being at the right speed, with a charge that gives the best groups, and if at all possible the best velocity stats in case we want to use that load at unknown distances.
A tight group at any distance is great, but they sometimes don't align to the best velocity stats. Make no mistake about it, when you see a great group at 100/200 with poor velocity stats, it generally falls apart at distance. The opposite is also true, when you see a tight group at 100/200 and it also has great velocity stats, it generally holds together at distance.
If you are trying for 600 yards, then play with the ballistics calculators and you will find that your velocity stats just need to be decent to keep the hypothetical vertical under control. In reality, there are many accomplished shooters that don't own a chrono and don't care about their velocity stats cause their groups are good. If you push a good 600 yard load to 1000 with a velocity stat that isn't state of the art, don't expect to play in the big leagues.
You can do fine with a decent velocity stat and short range testing to get out to 600. Your group testing will typically reflect your ability to tune, and that is dependent on good loading technique. None of us can predict what the rig will do from an undersampled charge ladder, so make sure you plan your next tests with a focus on your end goal. Pick the best groups and watch the speed too.
If there is no significant group size difference between two different loads, but there is a difference in their velocity stats, I would still encourage you to test both at 600 to learn the value of those stats. Good Luck.