Plotting doesn't work when all custom values are constant.

[gusinacio]

Taking #467 as a starting point, I wanted to have plots and removed .without_plots() option. I also updated to return iters * 42 instead of directly custom value.

This is returning the following error:

   Compiling benches v0.1.0 (s)
    Finished `bench` profile [optimized] target(s) in 0.86s
     Running benches/repro.rs (target/release/deps/repro-fcb679e095c2c534)
Gnuplot not found, using plotters backend
repro                   time:   [42 x 42 x 42 x]
                        change: [+0.0000% +0.0000% +0.0000%] (p = NaN > 0.05)
                        No change in performance detected.

thread 'main' panicked at /Users/gustavo/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/criterion-0.7.0/src/stats/univariate/sample.rs:31:9:
assertion failed: slice.len() > 1 && slice.iter().all(|x| !x.is_nan())
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

error: bench failed, to rerun pass `-p benches --bench repro`

Here is is the code:

use std::time::Duration;

use criterion::{
    criterion_group, criterion_main,
    measurement::{Measurement, ValueFormatter},
    Criterion, Throughput,
};

fn repro(c: &mut Criterion<MyMeasurement>) {
    c.bench_function("repro", |b| b.iter_custom(|iters| 42 * iters));
}

struct MyMeasurement;

impl Measurement for MyMeasurement {
    type Intermediate = u64;
    type Value = u64;

    fn start(&self) -> Self::Intermediate {
        return 0;
    }
    fn end(&self, i: Self::Intermediate) -> Self::Value {
        return i;
    }
    fn add(&self, v1: &Self::Value, v2: &Self::Value) -> Self::Value {
        return *v1 + *v2;
    }
    fn zero(&self) -> Self::Value {
        return 0;
    }
    fn to_f64(&self, value: &Self::Value) -> f64 {
        return *value as f64;
    }
    fn formatter(&self) -> &dyn ValueFormatter {
        return &MyMeasurementFormatter;
    }
}

struct MyMeasurementFormatter;

impl ValueFormatter for MyMeasurementFormatter {
    fn scale_values(&self, _: f64, round: &mut [f64]) -> &'static str {
        round[0] = round[0] as u64 as f64;
        "x"
    }

    fn scale_throughputs(&self, _: f64, _throughput: &Throughput, _: &mut [f64]) -> &'static str {
        panic!("unsupported")
    }

    fn scale_for_machines(&self, _: &mut [f64]) -> &'static str {
        "x"
    }
}

fn alt_measurement() -> Criterion<MyMeasurement> {
    return Criterion::default()
        .with_measurement(MyMeasurement)
        .warm_up_time(Duration::from_nanos(1));
}

criterion_group! {
    name = benches;
    config = alt_measurement();
    targets = repro,
}
criterion_main!(benches);

Criterion version: 0.7.0
Rust version: 1.90 stable