Providing an understanding how values change over time is one of the most fundamental functions of visualization, and the presentation of this type of information has evolved significantly since it was invented. Timeline visualizations serve as an aid to comprehension, allowing you to extrapolate and predict a trend. For example, if sales are consistently increasing by $100,000 per month over the last three months, it is easy for the human eye to imagine an increase over the upcoming months. Similarly, seasonality, such as the sales increases in the retail sector for the Christmas shopping season, becomes easy to see as a chart line rises and falls. Trends can be built into the charts as well as trendlines. Care must be taken when using trendlines to ensure that the user of the chart is kept aware of the fact that trends are derivations based on the data rather than actual measured values. Statistical predictions are another way of creating trendlines.