I think the word you want is model. The metric I came up with is just a way or measuring, evaluating, or tracking something. I happen to use ND/TD, because IMO it's the most accurate. I could just as easily used total cases, or total recoveries, etc., in any number of ways.
A model is a representation or simplified version of a concept, phenomenon, system, ..., or aspect of the real world.
A model predicts or provides accurate information when all factors affecting the outcome are well known and controllable. For instance, in determining stresses some structure may encounter based on known forces and strengths of materials that went into its construction.
I think the word 'model' is overused when applied to real world events such as hurricane tracking, global warming, and in this case, the spread of the corona virus when there are so many unknowns. Without constant updating, as many factors change, such a "model" is not be very useful. As we've seen in the news.
But you’re not giving your graph a fair chance.
I have to say, you're not the only one who looks at this as a proving of this model/graph/metric. Forget about actually trying to predict how fast the virus is spreading, which countries are having the best success, how many would have died if the current measures hadn't been taken, etc.
We're out to prove or disprove something. Well I'm not. I wanted to create a tool, this index based on daily deaths / total deaths, that would be useful in scoring how well a country or state was doing in fighting the spread of the Corona virus. It's proven very useful for that alone. As an additional benefit, it's usable in calculations to predict to some degree where we'll be in days or even weeks.
If one is only interested in proving if a projection was accurate or not, or to what degree, then there are the previous graphs I've posted which could serve as a historical reference to be used in determining exactly how accurate a previous prediction had been.
I already kind of do that with what I consider certain milestones. Back around the 6th April, or maybe even earlier, I had projected about 27,000 fatalities on 15 April. A projection I had run a couple days ago showed about 28,000 for April 15. Tonight I ran another projection that gave a result of 24,605 fatalities for 15 April.
What changed? What changed was the daily fatality index. Several weeks ago, my wife and I went to the grocer and maybe 5 out of 100 people were wearing masks. Two weeks ago we made a trip to the same grocer to replenish fresh food items and maybe 25 out of 100 people had masks. Today we made another trip to the grocer and more than 80 out of 100 people were wearing masks.
You can't model something that hasn't happened in living memory and which has so many unknown factors. It would be like trying to model the NY Stock Exchange and expecting accurate results months from now.
One can update graphs and plots and use what we call technical analysis to determine the general direction of movement, etc., but often real world events completely invalidate what we previously believed would happen.
So there...
Love the “so there...”