Original

During 1955 to 1975, the mortgage interest rate gradually increase from 5% to 8% and then followed by a dramatic increase from 1975 to 1985 to a peak of 20%. After that, it drops sharply from 1985 to 1995 to a bottom of 8% and then it growth up to 10% in 1995. However, it drops again from 1995 to 1998.

Edit

During 1955 to 1975, the average mortgage interest rate gradually increased from 5% to 8% and then followed by a dramatic increase from 1975 to 1985 to a peak of 20%. After that, it drops dropped sharply from between 1985 to and 1995 to a reaching the bottom of at 8% and then after which it growth up rose to 10% in 1995 1996. However, it drops dropped again from 1995 (10%) to 1998 (9%).

Final

During 1955 to 1975, the average mortgage interest rate gradually increased from 5% to 8% followed by a dramatic increase from 1975 to 1985 to a peak of 20%. After that, it dropped sharply between 1985 and 1995 reaching the bottom at 8% after which it rose to 10% in 1996. However, it dropped again from 1995 (10%) to 1998 (9%).

Comments:

• Remember to use the past when talking about actions that are finished in the past.
• I altered some parts not because they were wrong , but because it is better to use a little variety in your descriptions.
• It is good that you mentioned the peak and bottom, and that you described the trends without worrying about every little change.
  
• Note that the graph shows the AVERAGE mortgage interest rate.