The first line graph plots the percentage of homes that had a labor-saving electrical appliance between 1920 and 2019.

The first line graph shows the percentage of households that had a labor-saving appliance between 1920 and 2019.

The second line graph plots the average amount of time spent weekly on domestic chores per household during the same period.

The second line graph shows the average amount of weekly time spent on housework per household during the same period.

Nearly none owned a refrigerator in 1920, while four in ten % had a washing machine and three in ten had a vacuum cleaner.

"four in ten" as in a fraction is correct. However, as you added % sign it makes it confusing.
Almost no one owned a refrigerator in 1920, while four in ten owned a washing machine and three in ten owned a vacuum cleaner.

After that, however, refrigerator ownership underwent the largest increase, surpassing the other two after 1940 at 60% and reaching 100% in 1980, when it remained unchanged thereafter.

Your sentence is correct. You can also say:

Later, however, refrigerator ownership experienced the greatest increase, surpassing the other two after 1940 by 60% and reaching 100% in 1980, after which/when it remained unchanged thereafter.

The ownership of vacuum cleaners also experienced a notable rise and reached 100%, albeit much later, in 2000, when it remained at that level as well.

Vacuum cleaner ownership also experienced a notable increase and reached 100%, although much later, in 2000, it also remained at that level.

In contrast, the percentage of families owning washing machines, although starting at the highest value, only rose moderately and never reached 100%, but only about 75% in 2019.

You can also say: "In contrast, the percentage of households owning washing machines, although starting from the highest value, increased only moderately and never reached 100%, but only around 75% in 2019."