Axis lock#

Axis lock fixes one calibrated X or Y coordinate while you place manual points. It is useful whenever one coordinate is prescribed and the other must be read from the graph: sampling a curve at standard time intervals, reading Y at a sequence of X values, placing several points at one Y level, or using X/Y values from another dataset as a sampling schedule.

Enable a lock#

Open Axis lock in the graph toolbar. Choose whether to lock X or Y, then choose the value source. The toolbar displays the current axis, locked value, and progress. A guide line appears on the graph while the lock can resolve a valid coordinate.

Axis lock requires a valid calibration, a loaded image, and an active dataset. It is designed for Add/edit points manual placement; turn it off before using Trace curve.

Sequence#

Choose Sequence to generate values from Start, End, and Step. The step cannot be zero and must move from Start towards End. GraphX includes each value reached by repeatedly adding the step, including End when it is reached within normal floating-point tolerance.

For example, Start 0, End 10, Step 2 produces 0, 2, 4, 6, 8, and 10. A decreasing sequence is also valid when the step is negative and moves towards the End value. Sequence is useful for measurements at standard intervals, but it does not create points until you click the graph.

Values#

Choose Values to enter a custom list. Separate finite numbers with commas or new lines. GraphX preserves the supplied order and permits repeated values, which can be useful when the same X or Y coordinate intentionally occurs more than once. An empty or invalid list cannot be used as an active lock.

Dataset#

Choose Dataset to use the corresponding calibrated coordinate from another dataset. An X lock takes that dataset’s X values; a Y lock takes its Y values, in the source dataset’s current point order. The active dataset cannot be selected as its own source. If the source data changes, the lock resolves its values from the current source points, so review the progress if that dataset has been edited or reordered.

Placement and geometry#

With a lock active, GraphX converts your click to graph coordinates, replaces the locked coordinate, and converts the resulting graph point back to a source-image position. The other coordinate still comes from your click.

This matters on non-axis-aligned images. A constant graph X or Y is not necessarily a vertical or horizontal line on a rotated or sheared Fast calibration, and it can be projected across a Perspective-calibrated photograph. The on-canvas guide follows the actual calibrated geometry and is clipped to the source image.

Progression#

After each manually added point, Axis lock advances to the next value and the progress indicator updates. After the final value is placed, GraphX turns the active lock off automatically so normal unconstrained digitising resumes. The saved definition remains available to configure again.

Scope and limitations#

Axis lock constrains new manual point placement only. It does not constrain visual correction of an existing point or direct X/Y coordinate editing in Data points. There are no keyboard modifiers for it.

Trace curve is unavailable while a lock is active because a trace would otherwise advance through many lock values unexpectedly. On a logarithmic axis, every locked value must be valid for that scale, normally meaning positive. A lock may guide a position beyond the calibrated rectangle when the calibration can still map that coordinate, so inspect extrapolated-point indications after placement.