gg_ttest_between: Draw p-value between samples
In azzoam/ggBrackets: ggplot2 barplot significance test layer

ggplot2 layer to perform a 2-sample t-test and plot the p-value between two samples in a bargraph. Used in conjunction with gg_bracket_between().

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gg_ttest_between(data, sample_col, sample1, sample2, mean_col, sd_col, n_col,
  group_col = NA, group1 = NA, group2 = NA, equal.variance = T,
  dodge.width = 0.9, p_value_star = F, extra_y_space = 0, ...)

`data`	The dataframe used in the plot
`sample_col`	Column name of the samples in data
`sample1`	The first sample to be compared
`sample2`	The second sample to be compared
`mean_col`	Column name of the means of the observations. Usually the y-axis of the bargraph.
`sd_col`	Column name for standard deviations of the observations.
`n_col`	Column name for number of replicates in an observation.
`group_col`	Optional – Column name of the sample groups if applicable. Leave NA if there are no groups.
`group1`	Optional – The group to which the first sample belongs. Leave NA if there are no groups.
`group2`	Optional – The group to which the second sample belongs. Leave NA if there are no groups.
`equal.variance`	Optional – Boolean for whether variances are equal. TRUE results in a student's t-test. FALSE results in a Welch test. Defaults to TRUE.
`dodge.width`	Optional – The width of position_dodge() in the ggplot if groups are present. Usually 0.9.
`p_value_star`	Optional – Boolean for whether to plot stars representing significance levels or actual p-values. TRUE plots ''. FALSE plots p-values. '' = p-value < 0.05, '' = p-value < 0.01, '*' = p-value < 0.001, 'n.s.' = not significant.
`extra_y_space`	Optional – Numeric to adjust position of the bracket up or down from its default location.
`...`	Additional optional parameters/asthetics. e.g. color for font color or size for font size.