visualdynamics.plot.log_axis

log_axis

Log-axis tick labels that fit the axis they are drawn on.

pyqtgraph's log axis labels every tick from 1 to 9 in each decade when the axis spans few decades, and the first level of tick text is always drawn, however little room there is: two decades on a plot 100 pixels tall carried eighteen labels on top of each other. It also scales a log axis by an SI prefix, so a spectrum in N·s read '10¹' for 1e-5 beside a '×1e-06' on the axis label.

Here the labels are the densest set that fits: every mantissa, then 1, 2 and 5, then 1 and 3, then the decades alone, then every second, fifth or tenth decade. The room a label needs is measured from the axis font: a line's height on a vertical axis, the widest label's width on a horizontal one. Every tick from 1 to 9 is still drawn, unlabeled, while there are pixels for it, so the grid reads as a log grid. A span too narrow to hold two of those labels (less than a factor of two, say) is labeled at round values in between instead. A log axis takes no SI prefix: its labels carry their own powers of ten.

This applies to every axis in the process. The package makes its plots in more than thirty places, each through pyqtgraph's default axis, and how a log axis is labeled is a property of the axis, not of a plot, so the three methods are replaced on pyqtgraph.AxisItem itself when the plotting package is imported (install).

Functions:

Name	Description
label	A log axis's text for value: plain digits from 0.01 up to
choose	The mantissas to label and every how many decades, for a log
log_tick_values	Replaces AxisItem.logTickValues: the labeled ticks first, the
log_tick_strings	Replaces AxisItem.logTickStrings: the text of the labeled ticks,
update_auto_si_prefix	Replaces AxisItem.updateAutoSIPrefix: no prefix on a log axis,
install	Put these on pyqtgraph.AxisItem, for every axis in the process.

Functions:

label

label(value: float) -> str

A log axis's text for value: plain digits from 0.01 up to 10 000 ('0.5', '200'), powers of ten outside ('2·10⁻⁵', '10⁶').

Source code in src/visualdynamics/plot/log_axis.py

def label(value: float) -> str:
    """A log axis's text for `value`: plain digits from 0.01 up to
    10 000 ('0.5', '200'), powers of ten outside ('2·10⁻⁵', '10⁶')."""
    if PLAIN[0] <= abs(value) < PLAIN[1]:
        return f'{value:.4g}'
    mantissa, exponent = f'{value:.2e}'.split('e')
    mantissa = mantissa.rstrip('0').rstrip('.')
    power = '10' + str(int(exponent)).translate(_SUPERSCRIPT)
    return power if mantissa == '1' else f'{mantissa}·{power}'

choose

choose(low: float, high: float, pixels: float, room: float) -> tuple[tuple[int, ...], int]

The mantissas to label and every how many decades, for a log span from low to high (log10 values) drawn over pixels, each label needing room pixels from the next.

Source code in src/visualdynamics/plot/log_axis.py

def choose(low: float, high: float, pixels: float, room: float
           ) -> tuple[tuple[int, ...], int]:
    """The mantissas to label and every how many decades, for a log
    span from `low` to `high` (log10 values) drawn over `pixels`, each
    label needing `room` pixels from the next."""
    per_decade = pixels / max(high - low, 1e-12)
    for mantissas in MANTISSAS:
        marks = np.log10([*mantissas, 10])
        if np.diff(marks).min() * per_decade >= room:
            return mantissas, 1
    for step in DECADE_STEPS:
        if step * per_decade >= room:
            return (1,), step
    return (1,), DECADE_STEPS[-1]

log_tick_values

log_tick_values(axis: AxisItem, minVal: float, maxVal: float, size: float, stdTicks: list) -> list

Replaces AxisItem.logTickValues: the labeled ticks first, the rest unlabeled after them.

Source code in src/visualdynamics/plot/log_axis.py

def log_tick_values(axis: pg.AxisItem, minVal: float, maxVal: float,
                    size: float, stdTicks: list) -> list:
    """Replaces `AxisItem.logTickValues`: the labeled ticks first, the
    rest unlabeled after them."""
    low, high = sorted((float(minVal), float(maxVal)))
    if not (np.isfinite(low) and np.isfinite(high)) or size <= 0:
        return stdTicks
    candidates = [label(10 ** v) for v in _in_span(low, high, (1, 2, 5))]
    mantissas, step = choose(low, high, size, _room(axis, candidates))
    labeled = _in_span(low, high, mantissas, step)
    if len(labeled) < 2:
        # too narrow a span for round mantissas: round values between
        lo, hi = 10 ** low, 10 ** high
        spacing = axis.tickSpacing(lo, hi, size)[0][0]
        first = np.ceil(lo / spacing) * spacing
        values = np.arange(first, hi + 0.5 * spacing, spacing)
        labeled = [float(np.log10(v)) for v in values if lo <= v <= hi]
    axis._log_labels = {round(v, 9) for v in labeled}
    every = _in_span(low, high, range(1, 10))
    per_decade = size / max(high - low, 1e-12)
    if np.log10(10 / 9) * per_decade < 2.0:
        every = _in_span(low, high, (1,))          # no room for minors
    rest = [v for v in every if round(v, 9) not in axis._log_labels]
    return [(1.0, labeled), (None, rest)]

log_tick_strings

log_tick_strings(axis: AxisItem, values: Sequence[float], scale: float, spacing: float) -> list[str]

Replaces AxisItem.logTickStrings: the text of the labeled ticks, nothing for the rest.

Source code in src/visualdynamics/plot/log_axis.py

def log_tick_strings(axis: pg.AxisItem, values: Sequence[float],
                     scale: float, spacing: float) -> list[str]:
    """Replaces `AxisItem.logTickStrings`: the text of the labeled ticks,
    nothing for the rest."""
    chosen = getattr(axis, '_log_labels', None)
    return [label(10 ** v) if chosen is None or round(v, 9) in chosen
            else '' for v in values]

update_auto_si_prefix

update_auto_si_prefix(axis: AxisItem) -> None

Replaces AxisItem.updateAutoSIPrefix: no prefix on a log axis, whose labels carry their own powers of ten.

Source code in src/visualdynamics/plot/log_axis.py

def update_auto_si_prefix(axis: pg.AxisItem) -> None:
    """Replaces `AxisItem.updateAutoSIPrefix`: no prefix on a log axis,
    whose labels carry their own powers of ten."""
    if axis.logMode:
        axis.autoSIPrefixScale = 1.0
        axis.labelUnitPrefix = ''
        axis._updateLabel()
        return
    _stock_si_prefix(axis)

install

install() -> None

Put these on pyqtgraph.AxisItem, for every axis in the process.

Source code in src/visualdynamics/plot/log_axis.py

def install() -> None:
    """Put these on `pyqtgraph.AxisItem`, for every axis in the process."""
    pg.AxisItem.logTickValues = log_tick_values
    pg.AxisItem.logTickStrings = log_tick_strings
    pg.AxisItem.updateAutoSIPrefix = update_auto_si_prefix