Exactly.
So an iron meteor 500m in diameter would have a much higher MT rating than a 500m ball of ice (i.e., a comet). So you can't plot MT rating vs diameter. You have to do it vs mass.
I just noticed something else funny about this plot.
It has the annual event listed at 20 MT. But if you look at where that point lies on the horizontal axis, it is well below the 1 MT tick mark.
Also, if we had a 20MT event once a year, I think we'd notice it.
The bomb dropped on Hiroshima had a yield under 20 KT, which is 0.02 MT.
Here's the description of what a 20 MT nuclear weapon would do.
EFFECT OF 20 MEGATON NUCLEAR BOMB
What Would Happen if a 20 Megaton Nuclear Explosion on a City of 3 Million?
Ground Zero
Within 1/100th of a second, a fireball would form in every direction from ground zero enveloping downtown and reaching out for two miles.
Temperatures would rise to 20 million degrees Fahrenheit, and everything — buildings, trees, cars, and people - would he vaporized.
2 to 4 Miles from Ground Zero
The blast would produce pressures of 25 pounds per square inch and winds in excess of 650 miles per hour. These titanic forces would rip buildings apart and level everything, including reinforced concrete and steel structures. Even deep underground bomb shelters would be crushed.
4 to 10 Miles from Ground Zero
The heat would vaporize automobile sheet metal. Glass would melt. At this distance, the blast wave would create pressures of 7 to 10 pounds per square inch and winds of 200 miles per hour. Masonry and wood frames would be leveled.
16 Miles from Ground Zero
The heat would ignite all easily flammable materials -houses, paper, cloth, leaves, gasoline, heating fuel - starting hundreds of thousands of fires.
Fanned by blast winds still in excess of 100 miles per hour, these fires would merge into a giant firestorm more than 30 miles across and covering 800 square miles. Everything within this entire area would be consumed by flames. Temperatures would rise to 1400° F. The death rate would approach 10070!
Beyond 16 Miles
The blast would still produce pressures of two pounds per square inch, enough to shatter glass windows and turn each of them into hundreds of lethal missiles flying outward from the center at 100 miles per hour.
At 29 miles, the heat would be so intense that all exposed skin, not protected by clothing, would suffer third degree burns. Even as far as 40 miles from ground zero anyone who turned to gaze at the sudden flash of light would be blinded by burns on the retina and at the back of their eyes.
Within minutes after the bomb exploded 1,000,000 would die. Among the 1,800,000 survivors, more than 1,100,000 would be fatally injured.
Another 500,000 would have major medical injuries from which they might recover if they received adequate medical care.
Less than 200,000 people would remain without injuries - with very few doctors and with only limited medical facilities.
(top secret testing in the old USSR,USA and a few others.) 
