The sparklines on your episode list show the listen activity on each episode. When there is a meaningful number of listens on a recent episode in the past month, a sparkline will be rendered showing a summary of its listen statistics.
Sparklines are meant to provide at-a-glance data about which episodes are getting attention. Sparklines will not render if there is not enough data to show meaningful data.
Sparklines are not rendered with a scale relative to other sparklines that are visible. That is, a sparkline's maximum value is not equal to the maximum value shown on other sparklines; each sparkline has a separate range.