The basic problem is too many data points to plot (>1000). Generally the solution is to either reduce the number of distinct elements in your by clause (which maps to the number of lines) or to increase your timestpan width (which decreases the number of positions on the X-axis that have data) or both. For example, change span=1m to span=1h. Without seeing your actual search (which you should ALWAYS supply), specific help is impossible.
Based on the warning message on the dashboard, you're trying to plot more than 1000 points in the chart. You should be using some sort of aggregation command (stats\timechart\chart etc) to reduce the no of rows to be plotted and get more useful info. Could you post your query ?