I was wondering this, it sure seems that way for me, I was wondering what other people’s experiences have been.
In my experience it works this way; once I have one lucid dream it’s easier to have another, and then another, etc.
Lucid dreaming isn’t really that difficult after all 
i agree
Well… I used to have alot of LDs, but then I stopped lding for no reason, and now I don’t have them at all. So in my experience, it’s harder.
My guess, as with anything, it has to do with intentionality and focus.
I don’t mean to discourage any beginner lucid dreamers, but in my personal experience I’ve had numerous short, low-level-lucidity lucid dreams, and I still have trouble trying to have lucid dreams. I think motivation, confidence, and perseverance (all three of which I have trouble with) are the major determinants in having lucid dreams. I suppose lucid dreaming might become easier after the very first few lucid dreams that you have, but don’t expect inducing lucid dreams to be a walk in the park after the first one you have.
For the most part I think that is true. However, I would not say it is definitely not a diagonal line gradually increasing up as you progress. In stead you may experience a lot ov peeks and valleys along the way. The important thing is that once you have that first lucid dream it opens you eyes. You finally have proof that lucid dreaming is real and a hell of a lot easier than you thought it was. That in turn boosts your confidence. However, some get to confident to the point where they get lazy and don’t practice the skills. This may cause them to see a drop in lucid dreams so they go back to square one and practice again. This cycle may repeat itself many times. On the other hand their have been people who once that have that first lucid dream just skyrocket from there. It all come down to the individual, their attitude, and how much they practice.