Working with Date Axes

Financial charts and epidemiological curves often use Dates on the X-axis. Dates are just linear numbers formatted differently.

The Unix Epoch Solution

The easiest way to extract date-based data is to calibrate the axis using days or years, then convert them later. However, dates aren't perfectly linear (leap years, unequal months).

Best Practice Method

1. Identify two distinct dates on the X-axis (e.g., Jan 1, 2020 and Jan 1, 2024).
2. Instead of entering the date string, convert these to Julian days, or simply treat the first date as `0` and calculate the exact number of days to the second date (e.g., `1461`).
3. Calibrate X1 as `0` and X2 as `1461`.
4. Extract your data points.
5. Use our Date Axis Parser tool to add the extracted day-offsets back to your base date (Jan 1, 2020) to generate ISO 8601 strings.

Handling Trading Days vs Calendar Days

Stock charts often skip weekends. If you extract linearly across a stock chart, a gap over the weekend will look continuous, distorting the time scale. You must use tools like pandas in Python post-extraction to re-index the data against a business calendar (BDay).