Trace the index value of search/ interpolation points in guided tour optimisation

```
explore_trace_interp(
dt,
iter = NULL,
color = NULL,
group = NULL,
cutoff = 50,
target_size = 3,
interp_size = 1,
accuracy_x = 5,
accuracy_y = 0.01
)
```

- dt
a data object collected by the projection pursuit guided tour optimisation in

`tourr`

- iter
the variable to be plotted on the x-axis

- color
the variable to be coloured by

- group
the variable to label different runs of the optimiser(s)

- cutoff
numeric; if the number of interpolating points is smaller than

`cutoff`

, all the interpolation points will be plotted as dots- target_size
numeric; the size of target points in the interpolation

- interp_size
numeric; the size of interpolation points

- accuracy_x
numeric; If the difference of two neighbour x-labels is smaller than

`accuracy_x`

, only one of them will be displayed. Used for better axis label- accuracy_y
numeric; the precision of y-axis label

a ggplot object for diagnosing how the index value progresses during the interpolation

Other main plot functions:
`explore_space_pca()`

,
`explore_space_tour()`

,
`explore_trace_search()`

```
# Compare the trace of interpolated points in two algorithms
holes_1d_better %>%
explore_trace_interp(interp_size = 2) +
scale_color_continuous_botanical(palette = "fern")
#> map id to the x-axis
#> map tries to color
```