If a linear trend can be enabled in Chart Properties: Trendline option, the option: "Continuous" in General tab needs to be ticked
If the Continuous option is not checked, a discrete axis is used. With a discrete axis, there is one data point for each value, and there is no relationship between the values.
On the other hand, with a continuous axis, the data points are positioned according to their domain value, ie the dimension that is used.
Time is a typical example of a continuous axis. When you have a time dimension such as Year or Month, there’s a continuous interval with unbroken sequence. (Year can be divided into months, days, hours etc.) Also please note, with continuous axis, numeric values should always be used.
One example of the difference (please refer to the attached QlikView file): In the bar chart on sheet “Main”, there are 2 sharts using the same dimension & expression and both use linear trend line. The top one uses a discrete axis, and the bottom one uses a continuous axis. There are only values for month 2, 4, 6, 8 and 10 so in the bottom chart, there are no bars for months 1, 2, 5, 7, 9, 11 and 12. However, with the discrete axis (on top), it is not apparent that there are “missing” bars, because we do not have that relationship between the bars, and the bars are evenly spaced out.
For this reason, continuous values is considered to be a safer way to handle null values.
Using the discrete or the continuous axis will affect how the B value is calculated, ie where the linear trendline cuts the Y-axis.