Errors:  

- demonstrates the comparison between can be simplified.

- The population is in billions should be integrated into the first sentence.

Corrected version:

The line graph compares the populations of China and India from 2000 to 2050, measured in billions.

Improved version:

The line graph compares the populations of China and India from 2000 to 2050, measured in billions.

Errors:  

- was also at" should be "was.

Corrected version:

In 2000, the population of India was one billion, while China's population was 1.2 billion.

Improved version:

In 2000, India’s population was one billion, while China’s population was 1.2 billion.

Errors:  

-  After that" can be more specific..

Corrected version:

Subsequently, there was a steady rise in the population of India, as well as an increase in the population of China, which peaked at around 1.5 billion in 2030.

Improved version:

Subsequently, India’s population steadily increased, while China’s population also rose, peaking at approximately 1.5 billion in 2030.

Errors:  

- some 1.5 billion" should be "around 1.5 billion.

Corrected version:

The population level of China remained almost steady at around 1.5 billion in 2020, 2030, and 2040.

Improved version:

China's population remained nearly constant at around 1.5 billion in 2020, 2030, and 2040.

Errors: 

- year by year" is redundant with "continued to increase steadily.

Corrected version:

Meanwhile, India's population continued to increase steadily.

Improved version:

Meanwhile, India’s population continued its steady increase.

Errors: 

- This sentence is correct but can be slightly refined for clarity.

Corrected version:

In 2030, the populations of India and China equaled.

Improved version:

In 2030, the populations of India and China were equal.

Errors: 

- After 2030" should be more specific.

- peaked" implies a temporary high point, which may not be clear here.

Corrected version:

Following 2030, China's population began to decline, reaching 1.4 billion in 2050, while India's population reached 1.6 billion at that time.

Improved version:

After 2030, China’s population began to decline, reaching 1.4 billion by 2050, while India’s population continued to rise, peaking at 1.6 billion at that time.

Errors: 

- In summary, we can say that" is redundant.

- initial expectations" should be more specific.

- At the end" should be "By the end.

- Sentence structure can be improved for clarity.

Corrected version:

Overall, the results are quite different from initial expectations because India's population was significantly lower than China's in 2000, but there was a steady rise in the population of India. By the end, China's population was lower than India's population.

Improved version:

Overall, the results diverge significantly from initial expectations. While India's population was much lower than China's in 2000, it steadily increased, ultimately surpassing China by 2050.