There are several synonyms for the phrase "falling line", which could be used to describe a downward trend or decline in a particular data set. Some of the commonly used terms include "descending line", "dropping trend", "plummeting graph", "sliding curve", "dipping chart", or "descending slope". Each of these phrases conveys a similar sense of a downward trajectory or negative trend, and can be used in a variety of contexts, such as in financial reports, market analysis, or scientific data sets. Whether you use "falling line" or one of its synonyms, the important point is to accurately convey the direction and speed of change in the data.