How to find a tick on a plot?

How to find a tick on a plot? - briefly

Call the axis object's get_xticks() or get_yticks() to retrieve an array of tick positions and compare them with the desired coordinate. For exact identification, inspect the Tick objects via ax.get_xticklines() or use a Locator to target specific ticks.

How to find a tick on a plot? - in detail

Locating a specific tick on a graph requires direct access to the axis objects that store tick positions and labels. In most plotting libraries, the axis provides methods to retrieve these values programmatically.

In a typical Python environment using Matplotlib, the process follows these steps:

• Create the figure and axis objects with fig, ax = plt.subplots().
• Plot the data as usual, for example ax.plot(x, y).
• Obtain the numeric positions of the ticks on the x‑axis with ax.get_xticks() and on the y‑axis with ax.get_yticks(). The returned arrays contain the coordinates where ticks are drawn.
• Retrieve the textual labels that correspond to each position using ax.get_xticklabels() and ax.get_yticklabels(). Each label object can be queried for its text via the .get_text() method.
• If a particular tick value is known, locate its index in the tick array with np.where(ax.get_xticks() == target_value)[0]. The same approach works for the y‑axis.
• Once the index is identified, the matching label can be accessed directly, enabling further manipulation such as highlighting, annotating, or extracting the label string.

For R users employing ggplot2, the procedure is similar but relies on the ggplot_build() function:

• Build the plot object with p <- ggplot(data, aes(x, y)) + geom_line().
• Extract the built object: gb <- ggplot_build(p).
• Access the tick positions through gb$layout$panel_params[[1]]$x.range for the x‑axis and gb$layout$panel_params[[1]]$y.range for the y‑axis. The exact tick locations are stored in gb$layout$panel_params[[1]]$x.major and gb$layout$panel_params[[1]]$y.major.
• Labels are available in the same list under x.labels and y.labels. Matching a known coordinate to its label follows the same index‑lookup principle as in Matplotlib.

When working with interactive environments, such as Plotly, the tick information is embedded in the layout dictionary:

• After constructing the figure, inspect fig.layout.xaxis.tickvals and fig.layout.yaxis.tickvals for numeric positions.
• Corresponding labels appear in fig.layout.xaxis.ticktext and fig.layout.yaxis.ticktext.
• Direct selection of a tick can be performed by searching these arrays for the desired value.

If the plotting library does not expose tick data, a fallback method involves reading the rendered image. Optical character recognition (OCR) can extract tick labels from the axis region, while edge detection can approximate tick line positions. This approach is less precise and should be used only when programmatic access is unavailable.

In summary, the reliable way to identify a tick on a plot consists of querying the axis object for tick coordinates, locating the index of the target value, and then retrieving the associated label. The same principle applies across major visualization frameworks, with each offering specific functions to expose tick arrays and label collections.