In example implementations, a method is provided. The method collects key performance indicator (KPI) data of a plurality of network devices periodically. The KPI data of each one of the plurality of network devices is converted into a bitmap for each time period that the KPI data is collected. A number of the plurality of network devices that have exceeded a KPI threshold is detected based on the bitmap for the each time period. A determination is made that the number of the plurality of network devices exceeds a number threshold. In response to the determination, a time series graphical image of the KPI data for the each time period is generated based on the bitmap for the each time period.