No, it's not complete code (*), you don't show what's happening in the OnHint, OnHintPosition, OnHintLocation (you need only one of these...) handlers. The OnHint handler in reply #1, for example will crash when the mouse goes into the chart without hitting the data point, because then ATool.Series is nil, maybe even ATool. Always check for nil in these handlers.
(*) Rather than presenting such individual procedure it would be better if you'd upload a complete compilable project because then the compile and debugger would help me to find these issues. But PLEASE: Do not upload your complete project, separate the issue into a smaller project which makes it easier for me to reproduce errors. For uploading a project, pack the .pas, .lfm, .lpi, .lpr and possible data files into a common zip and use the "Attachment and other options" section of the forum. Do NOT include any files generated by the compiler, most importantly the executable, because there is an upload size limit.